Top-Off Reservoir

Using a Top-off Reservoir to Maintain your Hydroponics System

By cch2o

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Do I need a top-off reservoir?

The short answer is yes!  Having a top-off reservoir connected to your hydroponic system or Under Current will allow you to maintain accurate water levels and nutrient levels throughout the course of a growing cycle.

A top-off reservoir can also be used for fresh water storage.  This is helpful if you are using a nutrient doser or automation equipment.  Having a suitable amount of fresh water on hand will also allow you to do a full system water/nutrient change if needed.


Sizing your Top-off Reservoir

The top-off reservoir should be sized to be equal to your total systems volume plus 10% extra.  Example:  If you have a 90-gallon hydroponics system or Under Current a 100-gallon top-off reservoir would be ideal.Reservoirphoto2

A single top-off reservoir can be used for multiple hydroponics systems or Under Currents assuming they will all be feeding on a similar nutrient solution on the same veg/bloom cycle.   Example:  If you have 5 x 90-gallon hydroponics systems or Under Currents a 500-gallon top-off reservoir would be ideal.


Choosing the Right Top-Off Reservoir

Reservoirs come in a variety of sizes and styles from round to square and tall to short.  They are available from domestic and international manufacturers worldwide.  When selecting a top-off reservoir there are many things to consider.  Volume, dimensions and price will be major factors.  Freight costs may also become a major factor if ordering online or through a retailer.  Searching for a local provider will significantly reduce freight costs.

Some things to consider when choosing the right reservoir:

  • Overall dimensions: Will it fit in your grow space? Will it fit through the doorways to your grow space?
  • Volume: Will the volume be suitable to fill your entire hydroponics system or Under Current?
  • Materials/Quality: What is the reservoir made of?  Will it be suitable for clean water or nutrient solution?  Is it food-safe?  Is it UV resistant?  How long will it last?
  • Inlet/Outlet: Where are the inlet/outlet ports located?  What size are they?  What fittings or adapters will I need to connect this to my hydroponics system or Under Current?
  • Ease of Maintenance/Cleaning: How big is the access opening?  How easily can I drain my reservoir? Do I need a ladder or special tools to properly access? Will I be able to adequately clean the entire reservoir?
  • Additional Accessories: Does the reservoir include a lid, stand, bulkhead, drain valve or float valve?  Where will I source these parts?  How difficult will they be to install?
  • Price: How does the price stack up to available options?  Does the price include a lid and any other necessary accessories?
  • Freight Costs: If ordering a reservoir, what is the additional shipping/freight costs?  Can I find something locally to reduce these costs?

Our Aquifer reservoirs come in three variations: flat-bottom, cone-bottom and doorway styles.  We also offer FlexiTanks:


Aquifer Cone-Bottom


Aquifer Flat-Bottom




Aquifer Doorway

  • Aquifer Flat-Bottom Reservoirs: Are ideal in commercial spaces were floor space is of little concern.  They are heavy-duty, robust and durable.  They have the largest capacity and are the easiest to clean.  Available in 65-1000 gallon sizes.
  • Aquifer Cone-Bottom Reservoirs: Also called conical reservoirs.  These come standard with a metal stand raising the outlet off the ground.  The cone-bottom allows for a full drain out and easy cleaning.  These reservoirs are becoming very popular as tea brewers.  Available in 30-150 gallon sizes.
  • Aquifer Doorway Reservoirs: Are ideal in tight spaces were access is limited.  Doorway reservoirs are slimmer allowing them to fit through typical residential and commercial doorways.  They can also tuck up against a wall or fit in a closet to maximize grow space.  These units include an access port but are more difficult to clean.  Available in 250-500 gallon sizes.
  • FlexiTank Reservoirs: Require no tools for assembly, reduce storage requirements and take minutes to assemble. The FlexiTank Reservoir can fit where other reservoirs cannot and can be packed away & stored at the end of the season.  Available in 60-200 gallon sizes.

Important Reservoir Accessories

  • Lid: The reservoir lid covers the reservoir’s access point and keeps debris from entering the reservoir.  A reservoir lid may or may not be included, make sure you inquire before purchase.
  • Stand:  Some reservoirs are designed to sit on a stand; others simply sit on the floor.  Ask your retailer for stand options.  If you want your reservoir up off the floor you can use a variety of alternate methods including shelving, pallet racking or simple cinder blocks.   Having your reservoir up off the ground utilized gravity to aid in draining.
  • Bulkhead & Valve: Most reservoirs will include a bulkhead fitting but not a valve or adapter to your hydroponics system or Under Current.  A reliable valve is a crucial part of your reservoir plumbing.  This valve will allow you to adjust the flow of solution from your reservoir and attach hoses, etc.  Visit your local hardware store or a plumbing supply company for various valve/connection options.  All Under Current systems include a “Reservoir Adapter Kit” that will connect your top-off reservoir to the float valve in the system.
  • Float Valve: Having a float valve in your top-off reservoir will allow you to maintain the water volume.  The float valve will allow water to enter the reservoir up to a specific level; once it reaches that level it will automatically stop the flow of solution into the reservoir.  The float valve can be connected directly to a fresh water source or water filter.
  • Water Filter: Attaching a water filter directly to your top-off reservoir will allow you to store large amounts of pure fresh-water to be used in your hydroponics system or Under Current.  Starting with the purest water is an important step in maintaining healthy plants.
  • Dosing & Automation: A nutrient doser or fertigation/automation system can be plumbed directly to your top-off reservoir.  These units will dose, monitor and maintain desired nutrient and pH levels.  Newer dosing equipment can be monitored and adjusted remotely via a computer or smart phone app.  The staff at Current Culture H2O prefers Bluelab’s line of “connect” enabled meters and dosers.


Setting up your Top-Off Reservoir

 Reservoir placement is very important; water is wet and heavy!  A poorly supported reservoir could result in property damage, flooding and/or catastrophic crop loss.  When setting up your top-off reservoir it is important that you do so in a flat, well supported area with good access.

Placing your reservoir above the water level of your hydroponics system or Under Current will aid in draining and gravity fed top-off.  Use a manufacturer recommended stand or durable metal shelving like pallet racking.  Tall cylindrical reservoirs should also be strapped with metal chain or wire to prevent tipping in the event of an earthquake or natural disaster.


Attaching it to your Hydroponics System or Under Current

Once supporting and secured your reservoir is ready to be plumbed.  Install your desired valve (ball or gate) in the reservoir’s bulkhead fitting, this will alReservoir Attachmentlow for precise flow control.  From the valve you can install a hose directly to your hydroponics system or Under Current float valve.  If the top-off reservoir is connected to a float valve in your hydroponics systems or Under Current it


will automatically maintain a set water level in your system.  The top-off reservoir could also be plumbed to a pump and hose allowing you to transport solution to any area within your grow facility.


Filling, Mixing and pH Balancing your Top-Off Reservoir

Make sure your reservoir is clean of any debris or dust before you fill it for the first time.  When filling your top-off reservoir be sure to start with the purest water possible.  Using a reverse-osmosis filter will ensure you are starting with clean, low ppm water.

Constant mixing of the reservoir will keep the solution viable and consistent.  Mixing can be done in two ways:  either with a submersible water pump or an air pump/air diffuser.

  • Submersible Water Pump: If using a submersible water pump choose one with a GPH rating of at least double the reservoir volume (Example:  Reservoir volume is 100 gallons, water pump rated to at least 200 GPH).  Place the submersible water pump in the base of the reservoir or attach it to the sidewall.  This will allow solution to flow in a circular pattern.
  • Air Pump/Air Diffuser: Using an air pump/air diffuser will not only mix your reservoir but it will aerate the solution as well.   If you plan on using an air pump/air diffuser select an air pump that has a LPM output rating equal to at least one half the reservoir volume (Example:  Reservoir volume is 100 gallons, air pump rated to at least 50 LPM).  Also be sure to choose an air diffuser that is rated for the total volume of your reservoir.  Our Aqua-Pore Pro diffusers and diaphragm air pumps are a great fit for aerating large reservoirs.   Place the air pump up on a shelf above the water level and secure the air diffuser to the base of the reservoir.  Clean or replace the air diffuser periodically to maintain performance.

When pH balancing your reservoir GO SLOW, you can always add more pH adjuster.  After filling your reservoir with fresh water add your desired nutrients.  Take a pH reading and determine if you need to adjust the solution UP or DOWN.  The desired pH range for most hydroponics systems or the Under Current is 5.5 – 6.5.  Within this pH range all macro and micronutrients are available for easy uptake by plants.  Add a small amount of pH UP or DOWN, wait for the reservoir to circulate and stabilize, approximately 10-30 minutes.  Take another pH reading and determine if more adjuster needs to be added.  Repeat these steps until the solution reaches the desired pH range.


Using the top-off reservoir for Nutrient Steering

 If using a top-off reservoir correctly, you can easily maintain your desired pH and EC/TDS levels with little effort.  This will reduce daily system maintenance and make your job of feeding and irrigating much easier.

Typically the top-off reservoir should be balance to be the same or slightly stronger than the nutrient solution in the system.   As a simple rule: if you have week one’s nutrient solution in your system you would have week two’s nutrient solution in the top-off reservoir.  As plants feed, the nutrient solution is replaced by the slightly stronger solution from the top-off.  This “steering” will help to maintain consistent nutrient levels or allow the nutrient strength to slightly increase each week.

This practice of “steering” can also be helpful in maintaining desired pH levels.  If pH levels consistently rise in your system over the growth cycle, the top-off can be maintained slightly more acidic.  This opposition will gradually balance out pH levels as opposed to direct adjustment.  Using acid or base concentrates directly in your system can damage plants roots or create lock-out.

Ideally the solution in the system should stay balanced even as the plants use nutrients and water. This is a good indicator that minerals & water are being used in equal proportions.  In the event EC/TDS levels start to rise in your system this is a good indicator you’re running your nutrient strength too high. Conversely, if your EC/TDS consistently drops in the system you may want to increase overall nutrient strength.


Tips & Tricks

 Using a top-off reservoir for EC/TDS stability will translate into improved plant health and greater pH stability.  Observations and slight adjustments are the best way to dial your system.  Once you have found the “sweet spot” operation of the system becomes quite simple.reservoir Photo

 Use a submersible water pump or aeration pump with diffuser to keep top-off solution thoroughly mixed and aerated.  These will allow your metering equipment to provide an accurate assessment of your top-off solution at all times without error.  Regularly calibrate your metering equipment.

Avoid adding products to your top-off reservoir that cause foaming or contribute to bio-films.  These tend to cause pH instability and require more extensive cleaning.  Clean your reservoir regularly to ensure the highest quality nutrient solution is delivered to your system and plants.v

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