20 L weirdness

[Takingtulips]

So this is not an emergency situation but it is very strange and concerning for me.
I’ve had my 20 L up and running for about a year now it’s running kinda overstocked but I’m keeping up with the water changes and it’s pretty heavily planted. About a month or so ago I started loosing 1 fish at a time about every other week. At first I thought it was a coincidence because most of my fish come from petsmart so I thought it could be a genetic issue especially since it was individual fish of different species not the Entire tank or the whole school.
In order I have lost
1 Cory cat
1 platy
1 dwarf Gourami
1 Cory cat
1 dwarf Gourami
And now another one of my Cory cats is being lethargic at the bottom of my tank
I tested my water last night and it was at 0ppm ammonia 0ppm nitrite and somewhere between 0 and 5 ppm nitrate. I did a 20% water change just to be safe since it had been about a week since the last one and I have not been gravel vacuuming. I haven’t been adding fish to replace the ones I’ve lost so my school of corys is getting a little small but I don’t want to add more fish until I figure this out.

 

[ChrissFishes01]

How do the fish's stomachs look? Any symptoms before they die?

My bet (with the limited information) is probably internal parasites of some sort. Petsmart fish come from the same places as LFS fish do. The diseases are picked up in the tanks at the store - internal parasites would just leech energy from the fish until it basically starved to death, so they'd be pretty thin when they died.

 

[TheAnglerAquarist]

How many cories do you have?

 

[BigManAquatics]

Since so many aquarium fish are products of inbreeding, good chance they just starting to get old. Not many of those live near "life expectancy", especially platies, in my experience

 

[Takingtulips]

How do the fish's stomachs look? Any symptoms before they die?

My bet (with the limited information) is probably internal parasites of some sort. Petsmart fish come from the same places as LFS fish do. The diseases are picked up in the tanks at the store - internal parasites would just leech energy from the fish until it basically starved to death, so they'd be pretty thin when they died.

Half my fish look bloated and the ones dying do look very skinny. as far as I can tell there aren’t any symptoms other than that before I find them dead

 

[ChrissFishes01]

If the stomachs are pinching in, I might try and track down some General Cure. It's not necessarily invert safe, but should take care of internal issues.

 

[Takingtulips]

How many cories do you have?

I have 4 left in the tank including the one in the process of dying

If the stomachs are pinching in, I might try and track down some General Cure. It's not necessarily invert safe, but should take care of internal issues.

Thank you! I just dosed the tank with general cure. The only snail in there that isn’t a pest snail is an assassin snail but I’m not totally sure he’s still around I haven’t seen him in a hot minute.