This is for AutoPot growers choosing a tank, or wondering how long they can leave the one they have. AutoPot publishes which tank supplies which system, but no figure for how much water a plant drinks per day, so the days-between-refills number has to come from your own grow. Measure your system's draw once in veg and again in flower, then pick the tank whose usable volume covers the gap you want.
The short answer
- Plan on using two thirds of any tank. AutoPot says to refill with about a third left.
- Find your daily draw: note how many gallons it takes to top the tank back up after 48 to 72 hours, then divide by the days.
- Days between refills = usable gallons ÷ gallons per day. Use the flower number, since that's when draw is highest.
- Whatever the tank sits on has to carry about 8.34 lb for every gallon it holds.
What AutoPot pairs with each system
Smaller systems ship with the 12.4 gal rigid tank. FlexiTanks become standard at 6 pots on the XL and XXL lines and at 8 pots on 1Pot and easy2grow, and the FlexiTank Pro is an optional upgrade.
| System | AutoPot tank |
|---|---|
| 1Pot, up to 6 pots | 12.4 gal hard tank is sufficient |
| easy2grow, up to easy2grow 4 | 12.4 gal hard tank is sufficient |
| 1Pot XL, GeoPot, or XXL, up to 4 pots | 12.4 gal hard tank is sufficient |
| 6Pot XL | 25 gal FlexiTank |
| GeoPot 8 | 25 gal FlexiTank |
| 9Pot XXL | 25 gal minimum |
| 36Pot | 105 gal FlexiTank minimum |
| 100Pot | 200 gal FlexiTank |
These pairings say the tank can supply the system. They don't say how many days it lasts, which is what the rest of this article works out.
Usable volume: the two-thirds rule
AutoPot's instructions say to refill when about a third of the solution is left and never to let the reservoir run empty. So the working size of any tank is two thirds of its label capacity: a 12.4 gal tank gives about 8.3 gal between refills, and a 25 gal FlexiTank about 16.7 gal.
Size for the refill point, not the empty tank. AutoPot says never let the reservoir run empty, and never drag or reposition a tank once it's filled. A tank sized to the last gallon gets run dry on a busy week.
Measure your own daily draw
- Mark a level. Fill the tank and mark the waterline with tape. FlexiTank Pro tanks have capacity markings on the inside.
- Wait 48 to 72 hours. Keep the plants in the same growth stage, and leave the system alone.
- Refill to the mark with a measured container. Count the gallons it takes. This works in any tank shape, including the tapered hard tank.
- Divide by the days. Gallons added ÷ days elapsed = your system's daily draw right now.
- Repeat in flower. Draw rises as the plants grow, so the veg number will overstate how long a tank lasts late in the run.
| Step | Veg | Flower |
|---|---|---|
| Gallons to refill to the mark | 3 gal after 72 hours | 6 gal after 72 hours |
| Daily draw | 1 gal/day | 2 gal/day |
| 12.4 gal tank (8.3 gal usable) | About 8 days | About 4 days |
| 25 gal FlexiTank (16.7 gal usable) | About 16 days | About 8 days |
Run the same math with your numbers. If you want a week away in flower and your draw is 2 gal/day, you need at least 14 gal usable, which is a 21 gal tank or larger.
Tank types and sizes
The 12.4 gal hard tank is rigid, with a lid and an AutoPot stand. Standard FlexiTanks are collapsible and fold flat between runs. The FlexiTank Pro adds a reflective, light-tight skin, an anti-tip design, and capacity markings inside, and uses the same fittings as the standard FlexiTank.
| Tank | Filled size | Usable (2/3) | Water weight, full |
|---|---|---|---|
| 12.4 gal hard tank | 24" tall, 14.5" wide at lid, 11" at base | 8.3 gal | About 103 lb |
| 6.6 gal FlexiTank | 11.8" dia x 14.6" | 4.4 gal | About 55 lb |
| 13 gal FlexiTank | 13.8" dia x 20.9" | 8.7 gal | About 108 lb |
| 25 gal FlexiTank | 17" dia x 29.5" | 16.7 gal | About 209 lb |
| 25 gal FlexiTank Pro | 19" dia x 27.5" | 16.7 gal | About 209 lb |
| 60 gal FlexiTank | 25" dia x 31.9" | 40 gal | About 500 lb |
| 60 gal FlexiTank Pro | 23" dia x 33.5" | 40 gal | About 500 lb |
| 105 gal FlexiTank | 26.8" dia x 43.7" | 70 gal | About 876 lb |
| 105 gal FlexiTank Pro | 28.9" dia x 41.3" | 70 gal | About 876 lb |
| 200 gal FlexiTank | 35.5" dia x 46" | 133 gal | About 1,668 lb |
| 265 gal FlexiTank | 47.2" dia x 35.4" | 177 gal | About 2,210 lb |
Weights are water only, at about 8.34 lb per US gallon, and don't include the tank or nutrients.
Height, load, and placement
Every tank on this list has to sit with its tap at least 6 inches above the highest AQUAvalve; how that rule affects cycling is covered in why your tray isn't cycling. Whatever raises the tank has to carry the full weight from the table above.
The AutoPot stand for the 12.4 gal tank is 8.69 inches tall and rated for 300 lb, against about 103 lb of water in a full tank. For a FlexiTank on a stand or platform, AutoPot says to fit the inline filter further down the 1/2" pipe rather than right at the tank, or the pipe pulls on the tank material and can cause leaks.
AutoPot also recommends placing the reservoir outside the growing area. A bigger tank has a bigger footprint, so check the filled diameter against the space before you order.
Parts that fix this
-
AutoPot Hard Tank, 12.4 gal
The rigid tank AutoPot pairs with systems up to 6 pots.
-
AutoPot FlexiTanks
Standard and Pro sizes for when your measured draw outgrows 8.3 usable gallons.
-
AutoPot Stand for 12.4 gal Reservoir
Adds 8.69 inches of height under a full hard tank, rated for 300 lb.
-
AutoPot Reflective Cover for Hard Tank
Keeps light and heat off a 12.4 gal tank that has to sit near the grow.
Questions
Not with organics or additives in the mix. AutoPot USA says those can drift in pH in a standing reservoir, and has you clean the tank each time it's nearly empty, so a tank that takes weeks to empty holds that solution longer. On those programs, size the tank to empty on a schedule you're willing to clean on; the drift causes are in why your reservoir pH drifts.
AutoPot describes the FlexiTank Pro's reflective, light-tight skin as its option for bright, high-temperature spots. AutoPot's water target is 64 to 70°F. For the 12.4 gal hard tank, the reflective cover does the same job.
The sizing math is the same for any tank. AutoPot only recommends its own fittings, and any reservoir still needs a filter on the supply and its outlet at least 6 inches above the highest valve.
Start from the pairing table above, then check your measured draw against it after the first week. What else a No Tank kit leaves out is listed in what comes in every AutoPot system.