I reset my tank after breaking it down completely. Added some Miracle gro organic choice potting mix. Capped the soil with the old gravel substrate, it was rinsed first. Add some of of the old water plus some new
... Anyway about three days later the fish were in distress. Pull them out and back into the holding tank again with old water from before. I thought it might be do to ammonia. Check the that with a new kit I purchased this morning. The two tests did not measure any ammonia. It's a 45 gal. tank with a low population of very small fish.
This tank is co2 injected. I don't know what's wrong. Does any know what is happening?

Double checked your CO2 levels?

What symptoms of distress were your fish showing?

co2 is the same as before. Ph is running about 6
4-6.6. perhaps it's the volume of plants which was reduced not creating enough 02. That may be the answer. Thanks

I take it you're monitoring your CO2 levels through checking the pH? That's actually not the most accurate method, as there are so many different variables that can cause pH swings besides just CO2.

I always recommend people get drop checkers with calibrated 4 dkh solutions in order to get more accurate CO2 levels.

There also may have been an ammonia and/or nitrite spike in the tank before you realized and were able to check the levels.