Pond Boss Forums GENERAL QUESTIONS Water chemistry Questions on keeping fish alive in my Aquaponics

I have an outdoor Aquaponic system that I set up this year. So far I have plants in it but no fish yet since I've been waiting for the filter to cycle and start properly removing ammonia and nitrites. I've been manually adding a small amount of ammonia daily to feed the filter bacteria. I think it's ready to add fish, but have some questions to ensure I don't kill them.

The system is a 250 gallon IBC fish tank, and has around 350 gallons of water total counting what's in the grow beds and sump tank. The plan is to add 25 fingerling tilapia once the filter can handle it, let them grow over the summer, then harvest them and shut down the system once it gets too cold.

My questions are:

1. The pH in the system swings a lot. It tends to drop fairly quickly when left alone. I test pH daily and bring it up as needed. When pH drops to 6.0 I add 1 tbsp of calcium carbonate or potassium bicarbonate, which brings it up to 6.6. Then, over the course of 48 hours or so it gradually drops back to 6.0 at which point I raise it again and repeat the cycle. It's been steadily like this for over a month. Will this amount of pH swing harm the tilapia?

2. Can anyone give me an estimate of how many ppm of ammonia 25 fingerling tilapia will raise 350 gallons of water each day? My biofilter is currently eating through around 0.5ppm of ammonia daily. Is that enough?

3. I need to have an electrician work on the power in the barn where the system is plugged in. I didn't plan very well and will likely have fish in the system by the time he can come. The power will have to be off for a little while while he works. So... 25 fingerling tilapia in a 250 gallon tank. Water temps stay between 80-90. What's the maximum safe time I can leave the pump off (so no filtration) assuming that I leave an aerator going? What's the maximum safe time to leave everything off (no filtration, no aeration)?

Thanks for any help you can give!

A 2" fish will produce about 0.45 mg of Ammonia per day.This amount disproportionately increases as the fish grow. A 4" fish will produce about 3.5 mg of Ammonia per day. Your biofiltration must be capable of handling the Ammonia level produced by he final weight/size of the fish at harvesting.
Any pH swing over 0.5 could eventually cause problems.

You can help stabilize your pH at a level up to 8.4 by using pelletized Dolomitic Limestone (calcium magnesium carbonate) Look for a brand with higher calcium than magnesium, though all will do fine and break down slowly over time to keep your acidic water buffered. Just add the Ag Lime anywhere in your system where water flows over a large surface area of lime, or create a separate buffering tank with heavy aeration similar to the biofilter tank.

What type of "ammonia" are you using? many household ammonias contain other chemicals that will kill your bacteria, not start it. Simply taking whizzes in your water work darn well to create the bacterial colonies....

25 fingerlings in 250 gallons would be happy for over a week in your water....IF not feeding, or recently fed (within 6 hours)

Thanks, everyone!

Rainman, to verify, you're saying they should theoretically be ok even with no filtration or aeration for up to a week? If so then that's great news. I can't imagine needing to shut off the power for more than 6 hours or so, tops.

I've tried various things to buffer the pH drops. At one point I brought the Total Alkalinity up to around 150 but it always gets eaten away fairly quickly and then the pH drops again. There must be something in there that's fairly acidic. I've been buffering it back up every couple of days for over a month now and it doesn't seem to be slowing down.

I have a sock full of crushed oyster shells in there. It isn't doing much. Do you think I'm better off adding more oyster shell, which I have readily available, or would the pelletized Dolomitic Limestone be better? Any thoughts on where to find it?

If the fish are "fingerlings" (under 4"), they would be fine for days! Even 1.5 pound adults in that volume of clean water would be fine for at least a full day, even without aeration.

You wouldn't have to worry unless your stocking density were approaching .5 pounds of fish per gallon of water....25 fingerlings in 250G is probably <1# per 250G

What is your source for the water?

Thanks, Rainman!

Bocomo, my water source is tap water left in the sun to dechlorinate. I top the system off with a few gallons per day to offset evaporation and transpiration from the plants.

The tap water pH is 7.8 with a Total Alkalinity of 100.

Ack, pelleted Limestone is at almost all stores that have a garden dept...wally world, big box places, crazy joe's hardware store....

Gradually expose a larger and larger surface of pelleted lime every few days till you get to the desired pH

Ack, your pH in your system goes to 6.0, but your tap water is 7.8 with Alkalinity of 100???? Something isn't adding up! What in your system is so acidic??? Check Hardness...if that is also near 100, your tap water is well buffered and something in your system is leaching a lot of acid to drop pH to 6.0 in just 2 days.

There may be dead/stagnant areas that are generating lots of hydrogen sulfide...your settling/solids tank?

Yep, crazy isn't it? I've discussed this at length with people on an aquaponics forum and their best guess matches yours - that there is leftover decaying organic material in my grow media that's producing (or produced) hydrogen sulfide. The grow beds and hydroton sat all winter and I didn't do much to clean out old roots, leaves, etc.

I did a test where I took out a few pieces of the hydroton (expanded clay balls used as growing media) and left them in a water bottle. The pH of the water in the bottle dropped over the next few days. So it's something in the hydroton. Hydroton is supposed to be pH neutral but obviously mine isn't, or has somehow been permeated with something acidic.

Rather than rip out all of my plants, clean out and rinse the beds and grow media, and start over I decided to just stick it out and keep buffering pH until I neutralize all of that acid. It's been over a month with little progress, so I'm starting to second guess that decision. But I'm too far in now to change course and still have any growing season left, so I'll stick it out for this year and clean it out over the winter.

I'll check hardness the next chance I get. I'm guessing that the system water hardness will be quite high since I've been buffering the pH up every other day with Calcium Carbonate.

And I'll definitely pick up some pelletized limestone. Thanks!