Lots And Lots Of Stuff Gone Wrong

So this is my second post since joining here, I believe I am still dealing with the same problem. This is also my first tank, so some of these problems could be related to that. The biggest problem is that my fish (3, then 2 and now 1 platy) have seemed to be infected with some sort of internal parasite. I bought these fish to cycle the tank (a 20 g, I researched before doing this and they have seemed fine until now) and they never seemed stressed. After I bought them, I started to notice that they had white, stringy poop mixed with their normal (same color as food) poop. I didn’t know it was abnormal at the time. About a week and a half after buying the fish, the largest male (they were 2 boys and a girl, I misjudged the gender on one) began to seem lethargic, hid, and didn’t want to eat. 5 days later, he died. Before he died, I looked up their symptoms and it seemed they had parasites, so I began treatment with API General Cure for Parasites. Since the others seemed fine, I finished the treatment and continued on with their care as normal. The tank appeared to have cycled by the time he died so it may not be related to that. Then, the other 2 fish began to get sick with the same symptoms, although they are for longer than the first one did. I began to treat them again 4 days ago, and the smaller male died as I wrote this. I would really like to save the last female (and the cycle), as she now has clamped fins as well. I did notice that in the end, the small male had very pale, almost bluish gills. They were normally a bright pinkish color because he was a very light yellow. Does anyone have any ideas as to what to do (and what is wrong?) Also, when will it be safe to add fish? The other problems (white fuzzy stuff on gravel and brown algae on decorations) I believe is because of overfeeding. The tank has been set up for a little over 3 weeks now. Please help ASAP, let me know if there is anything I may have missed. Thanks for your help!
Tank:
20g long
Aquaclear 30 filter
No ammonia or nitrite
Nitrates are pretty low (about 20)
pH is around 7.5
Water is very hard (can’t remember exact numbers)
Fish are fed with Omega One flakes (don’t know if that matters or not)
Tank is not planted
Inhabitant: The lone survivor, a female Platy

 

The symptoms definitely sound like internal parasites. How long have you been using the API treatment? The blue gills almost sound like an ammonia issue, but your parameters look good. In terms of the algae, you are probably correct in identifying overfeeding as the source. Do you have any cleanup crew in your tank to start eating the algae? How often do you do water changes?

 

Are you doing water changes? I’d start with 50-70%.
Before the water change, get your water parameters and post them. After the water change wait about an hour and then test parameters. Post those too. Also test your tap water.

 

Since you have already tested the water and everything is showing as normal it sounds like your fish either has both a bacterial and parasite infection or the parasites may have immunity to the specific Medication.

If you are able I would try to get ahold of either kanaplex or paraguard. Both medications treat a broad range of issues.

 

I do not have any cleanup, the platy is the only inhabitant of the tank. I try to do a 20ish percent water change every week, I’m still trying to get a schedule for tank maintenance together. It’s pretty hard for me to do larger water changes since I don’t have a python water changer and my gravel vac doesn’t work well. I plan on getting a nerite or two for the algae. I will try some new meds and will keep y’all posted