Odd Swimming Behavior

[JellyKat]

My sunset platy, Solana, is not her usual self this morning. She is grazing the bottom of the tank and did not even come up for breakfast when she is usually the first one there. Instead she is hiding in the crawl space between the heater and the glass. I see no ich spots on her, nor are any of her fins clamped to her, nor do I believe she is pregnant as she gave birth two weeks ago and is not nearly as big.
Her tankmates include a green lantern named Azure and the surviving fry in the breeders net.
My tank is 5.5 gallons which I understand most aquarists think is small but I don't plan on housing more than three fish here at a time and ONLY FEMALES. I change the water weekly, not with tap but purified drinking water that has been treated with bacteria supplement. The temperature is at a steady 78deg, and I ran a pH test this morning which says my tank is at 7.2.
During my last water change I added a live plant, java fern, which they were curious about at first but pay no more attention to now. This plant is releasing bits of oxygen everyday and the bubbles collect underneath the breeders net so I don't think my water is under-aerated.
I think I have all my bases covered for a healthy tank but something is wrong when your fish goes from turning backflips to lying low. Any advice/suggestions????

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[JellyKat]

Both my adult platies have passed.
I removed the java fern since I think this was too a drastic change to the environment and will let the tank cycle a few days before trying to bring anyone else home.

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[Gary R]

Hi JellyKat

Sorry to hear that you lost both your platies, I don't think it would be any think to do with the java fern plant

Your PH to me is a little low I would say keep it between 7.4 & 7.6 and I think that with your readings being at 7.2 could be with the water changes that you are doing.....how much water are you replacing each week ? as replacing to much would of upset your fish as well.