Dying platys- Unsure why

Hello! I have had my 20 gallon tank for about a year and I have killed a ton of fish. I made a ton of newbie mistakes and I'm starting to learn what I did wrong and what I need to do.
Anyway, my 20 gallon tank, in it I had 3 female Hybrid Platys, 3 different male platys (which never showed any interest in the females.), Three Female platy fry, 3 very tiny endler guppies, a cobra guppy, two peppered corys (one male, one female) a black molly, and a golden Chinese Algae eater. All my plants are fake.
3 weeks ago 3 of my endler guppies died, didn't know why.
2 weeks ago 2 platys (male red tiger platy and female metallic blue) died. The male seemed very lethargic and just sat at the back of the tank.
last week my 2 female hybrids seemed to be sick, both were lethargic and sitting at the back of the tank or at the water line in one place, fins clamped, but I couldn't notice any physical change in appearance. both stopped eating and eventually died.
I tried to figure out what was wrong, tried asking other forums and no one could help me.
I had no idea what flashing was until now, but all of my fish (except the corys and algae eater) have been doing it for a while...
Looked at many websites and apparently lots of platy owners had this problem and their fish eventually died because no one could help them.

Now my red tailed dalmatian platy is clamping his fins and becoming lethargic too.... when I fed them a few hours ago he seemed fine. I'm not sure what's happening... suggestions?

Tested the water and it reads:
Nitrate- 40
Nitrite- 0
GH- 75
KH- 180
PH- 7.8
Ammonia- .25

 

It sounds like your tank is, or was overstocked. If there's ammonia and nitrates, you basically have too many fish. You should perform a largewater change, as your nitrates are a bit high. The fish are most likely sick due to chronic stress because of the tank being overstocked and to many nitrates. I'm not sure what they have though. A picture would help identify it. For now, try to get maintain very clean water with frequent water changes.

 

Welcome to FishLore!

I agree that is seems that you are or were overstocked (I'm can't quite figure out what is in your tank right now) and that your fish are suffering from water quality issues. All the symptoms you listed can be brought on by poor water quality (i.e. ammonia, nitrites, high nitrates)

There are several reasons that you could have ammonia and nitrates at the same time. One as mentioned above is due to being overstocked. When overstocked yoor fish can be generating ammonia faster than the bacteria can process it. Usually you will develope enough bacteria to handle all the ammonia, but your stocking may have been such that there wasn't enough room in your filter media to hold all the needed bacteria.

Another reason for having ammonia and nitrates could be that your tank was cycled and for what ever reason you lost your cycle. Things that could make you lose your cycle would include changing your filter media (you should never change the media unless it's totally falling apart, just rinse in a bucket of used tank water during water changes), or adding meds that would kill off the bacteria in your filter.

BTW what test kit are you using?

Good luck! and feel free to ask more questions.

 

Ok, I did a 50% water change and vacuumed the gravel. the water levels are still the same.

I use Jungle Quick Dip 5-in-1 test strips and Jungle Quick Dip Ammonia Test Strips.

The only fish in my tank now are 2 male black bar endler guppies (moved in from my 5 gal), a male cobra guppy, a male red tailed dalmatian platy, a male sailfin platy, a female black molly, a female silver molly, one male and female peppered cory, and a chinese algae eater.

I have a Fluval C3 HoB filter, when you change the filter media, you can change like 1 of 5 different things. its supposed to make it easier on your fish and tank.

How often should I do water changes and for how long? I usually just change 40% once a week.

This is what my hybrid platy looked like before she died. this is how my other two platys look now.

 

Test strips are notoriuos for being inaccurate. I would highly recommend you invest in a good liquid based test kit like the API Master Test Kit. It is more expensive up front, but it is very accurate and will perform 100's of tests.

It sounds like you have the filter situation under control. but I will say that any time you replace filter media, you run the risk of going into a minI cycle, since there is only enough bacteria in your filter for the amount of ammonia (any additional bacteria would starve off). So if you remove and filter media, you suddenly have less bacteria then needed.

I'm not familiar with your filter, but my guess is that it has ceramic bio media (rings or stones). These should never be replaced, even though the directions probably tell you to.

So I would just keep up with the daily 50% water changes using Prime or some other detoxing water conditioner until your back to 0ppm ammonia and 0ppm nitrites.