Help! Fish dying, I don't know what's wrong.

[Aerowyn]

For background: in this post, I am talking about a 20 gallon tank, with (as of writing this) 6 bentos tetras, 5 neon tetras, a cardinal tetra, 2 trilineatus cories, 3 amano shrimp and 1 nerite snail. This tank is an upgrade from a 10 gallon and has been running a few months but the media is from the previous tank, and it had been running for almost 2 years. Up until yesterday, I have lost, over the period of about six months, 4 cories, a betta and 3 gouramis, all without any kind of warning signs... just woke up one day and they were dead (one at a time, sporadically over the months). I always test my parameters when these deaths happen and they've never been off, the worst I've had is ~30ppm of nitrates and that was only once, the rest of the time nitrates were under 15 and ammonia/nitrites are always 0. 


So, cut to yesterday - I had to euthanize a neon tetra because it was doing very badly and had all the symptoms of neon tetra disease (It had a patch of skin with no blue or red, its spine had kinked and there were a couple of bumps on its back, it was super skinny and not able to swim well, and it was not eating or schooling). This came on VERY quickly... I only noticed maybe 2-3 days ago that it was looking a tad sickly, whatever was wrong with it progressed very fast.

Now a second one has it by the looks of his body, nearly overnight he’s gotten a bit deformed and he’s losing colour (also a patch of skin where there should be blue and red). Two of the other neons look like they’re getting tail rot, but do not have any discolouration.

Also this morning, I lost a bentos tetra (who up until now have been completely fine, and the dead one had no symptoms of anything just yesterday), and now, one of my cories is on its side, gasping for air. 

My parameters current are good: ammonia 0, nitrite 0, nitrates ~15 (water change day tomorrow so those will be gone).


I don’t know what to do, or if I should dose some medication, or? I’m baffled. There seems to be multiple issues but I have no idea where to start. The only medication I currently have is Paraguard, but I can swing by the pet store tomorrow if need be.  Please help ;_;