Discus Dying

[DFWDiscus]

I have been keeping fish for decades but recently got my first batch of 2.5" Discus from a breeder. They took an extra day to get here and werent well when i got them, but they all lived long enough to make it into the planted, cycled 60 gallon 48"x12" tank with 1/3 gravel and 2/3 sand substate with only a couple of cardinal tetras and hachetfish for company. One of the 12 was dead the next day, but the rest lived and seemed to thrive.

After they had become established, i added some similar sized angels, small corys and a dwarf pleco from another tank to complete the community (way less than 50 in of fish and i plan on removing angels as they pair, which two have, and some discus as they get bigger)

No Ammonia or nitirite, about 40ppm nitrate, ph 7.2 KH 3 GH 8, Temp 82 i keep it pretty stable with buffer and some distilled water mixed in the tap, the pH has come down from 7.6 over the corse of a couple months since i filled the tank for the first time.

Everybody seems to get along well and be able to get enough food. Everybody looks great except some of the discus (the smaller ones, it seems) began to get listless and not come up for food. This started several days ago, and i upped my water changes to 20% every day (i had been doing most days to at least several times a week before, but now its the first thing i do when i get up every day.

This morning, two fish that had been looking well were dead. I changed the water. Now 12hrs later another one is unable to stay upright, but looks otherwise normal.

Is the nitrate at 40 enough to kill them? There's some in the tap water, so it's hard to get it down much lower than that.

Did i overfeed? I'm trying to keep them fed so they grow, but maybe the little guys had access to to much food and killed themselves. I didn't even know that was possible, but i have read that it is. I feed a varied diet of flake, pellets, freeze dried tubifex, brine shrimp and plankton and a couple times from a very reputable LFS.

Please Help! The attached photo is of the newly sick fish, who looks normal other than he can't swim and is sitting upside down on his face...

[GeauxDiscus]

I see no one else replied, but the answer is very simple (for anyone else who can learn from this): Discus cannot tolerate the same bacteria or parasites as many other aquarium fish. They aren't equipped for it, because they evolved in tannin-filled waters. If you put them in a tank with any other species, you're rolling the dice. It may turn out okay, but it may end in disaster. I'm truly sorry for the experience you had to go through - I learned this the hard way also.