IMO, you're doing radical water changes. 50% once a week, though may be effective on lowering nitrates, the downfall is that it's very stressful on the fish. You have hardy fish for the most part with the tetras being the most sensitive you have in that tank.
Stick with the PimaFix. Follow the water changes recommended on the bottle during treatment. Your nitrates are low so you should be able to wipe out whatever is bothering the platy in a short time. If there is no improvement after 2 weeks, switch the medication to something else. Once you are done medicating, place in fresh carbon and start a routine water change of 10% once a week instead of 50%. This routine will allow for a more stable environment. There are only a few things we test for, but water chemistry is loaded with all sorts of stuff not tested for. Large sudden changes can cause shock. Shock can lower the immune systems in fish and make them vunerable to all sorts of things.
FYI, feeding routine to prevent overfeeding is to give the fish a time limit per day. Whatever they can go through in two minutes per day. If you feed them more than once a day, then divide the time period accordingly. You can even skip a day or two out of the week. I'm not saying you are over feeding. I wouldn't know. One person's pinch is another's handful. Know what I mean?
Keep me updated on the platy
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We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.
Nov/2004
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