This is long and it spans many months, but help please if you have a moment! I think I have created a big mess and I don't know how to fix it. I had a tank with 2 mollys and 3 platys. I stayed on top of maintenance, nitrites and ammonia were always 0, I got the nitrates to about 20 and things were good until I had filter changes. We have well water with no nitrates, it is very hard (from what I Read this is good for these fish), and ph is naturally 7.3. The water isn't treated so no chlorine or additives. Every single time I changed the filter (only when the light on my filter would blink, maybe every 6 weeks), it seemed 2 platys would stress, one would get an ich spot or two, the other would go into hiding. I thought maybe I was removing too much good bacteria with the filter changes so I kept a piece of the old filter in with the new one the next time I had a change. The ick came back on this platy so I treated the tank with API fungus cure. I am guessing after that my tank may have cycled again? I have always done plenty of water changes, about every 4 days, and I would treat the new water with stress coat. I do get brown algae a little which the fish love to eat. Anyway, one molly had fry, I found two in the filter and decided to keep them. I knew this meant a larger tank size but figured I would wait until they were a little bigger to upgrade. I noticed that while the ammonia and nitrites were always 0, the nitrates were creeping up, and I struggled to keep them around 20, I was doing very frequent water changes and I began to use Prime also. I had some readings of 40 and stayed with water changes with the plan to upgrade the tank. The fish seemed happy enough, the two baby mollys were about 2 months old when I noticed that one of my platys was starting to look thin ( around this time I got a bigger tank and started to slowly fill it). This platy seemed happy, ate normally, just looked small. This went on about 3 weeks until about a week and a half ago when I noticed one adult molly had 2 ick spots and the platy was just looking very thin, so I treated the tank again with API fungus cure as I figured this was bacterial or fungal and probably because the tank was overstocked. I had put some of the old tank water into the new one to help cycle things. Now I realize this must have been stupid because the medication probably killed my good bacteria and if the meds didn't kill the actual problem I may have infected the new tank. The skinny platy ended up dying . The other fish seemed ok otherwise, just two ich spots on one platy but that was it. I noticed the baby mollys gills were moving fast, no other symptoms, but I moved them to the new tank as soon as I thought it was safe. The readings on the new tank were 0 for ammonia and nitrites and 20 for nitrates, I thought we were all good. I moved one of the adult mollys too just to lessen the load in the smaller tank which now had 1 molly and 2 remaining platys. Two days later the two baby mollys were dead in the new tank. They never showed ich spots or any other symptom, just looked stressed with clamped fins and hung out towards the bottom before they died. They actually died quickly if you ask me, I thought maybe the stress of the meds in the old tank and then moving to the new tank was too much for them. I moved the rest of the fish to the new tank, all seemed good, they were all eating, but then today I notice the first adult molly I moved to the big tank looks very stressed now, clamped fins, and while she was eating fine this morning doesnt seem to want to eat. The other molly seems stressed too, hanging by the heater. The 2 platys now are fighting and one is showing an ich spot. I have no idea what I did, if they are stressed from the new tank, from the meds, or if they are all sick. They have all had different symptoms, but none of them have many symptoms at all. I wonder if there could be parasites? My concern is now I have infected the new tank with parasites, but I am worried if I treat them with another medication it will be too much stress for them, but if I dont treat they will die of whatever is bothering them. I still have the heater on in the old tank, I have no idea if I should put them back in there? I also put the old filter in the new tank as well as the decor but I could put that back if the old tank might be better for them. If there is any advice I would appreciate it. I feel horrible, I am scared they are all going to die, and I don't know the best way to handle this. I lost 3 fish in the past week but it seems from different things. What would you do in this case?