Okay, some more information might be required here, so I came to my PC to type it out instead of using the android app.
CURRENT SETUP :
20 gallon long
Fluval CO2 system (less than one bubble per 3 seconds at the moment)
20g QuietFlow Aqueon Filter
Preset Heater (78 degrees)
Several plants (Primrose, Amazon Sword, Anubis Fern, Cryptocoryne)
Fluval Ecocomplete Substrate and Gravel mixed
Ohko Dragonstone rock
8 week nitrogen cycle performed prior to establishing any fish within the tank. Introduced 6 zebra danio at first and waited about a week. Then added 5 tetras / 5 sunburst platys. One platy got stuck in a rock and died a few days after, so I moved the rocks around and planted the new primrose and the cryptocoryne, did a few tests and everything came back properly (Ammonia was at 0ppm, Nitrite at 0ppm, PH at 6.8, Nitrate around 10ppm. )
After the tests, I decided to replace the fish, but noticed tetras were getting nipped a little, so increased school size a little. Total Tank population at 8 neons, 6 zebra danios, and 6 sunburst platys. I also used this point to go ahead and get a CO2 system set up and set low to 1 bubble per 3 ish seconds.
Everything fine for a few days, it seemed until tonight. After feeding, found platy laying on substrate with small red dots I don't see on any of the other fish. No evidence of fin nipping or any territorial pressure from danios / tetras. Possibility for ammonia burns?
Retested water. Ammonia at 0.50 ppm (!!) , Nitrite at 0ppm, Nitrate around 20ppm, pH still at 6.8 holding steady.
Hopefully this fills in the info. Ammonia burns?
Any advice on steadying this process while the tank bolsters the biofiltration a bit more?