Perfectly Healthy Kissing Gouramis Dropping Like Flies

[toxik]

Here were the stats of my water when the last fish died.
36g
Amm - 0
Nitrite - 0
Nitrate - 40

We figured it was the large amounts of nitrates. Her tap tested at 20ppm.
We did a change with RO and it dropped to around 10-20ppm.
We bought a new kissing gourami, it hid for the first day, came out and swam for the next two. This morning before the lights came on he was swimming. After the lights came on he wasn't moving but still breathing, eventually he died.

As far as I know they eat. They eat the algae wafers i put in and occasionally eat the flakes. No other physical ailments as far as bumps, fin rot, cuts, bruises, etc.

We've lost 5 so far, and have not lost anything else.

Here's the stock

6 asst. corys
4 kuhlis
about 10 or so tetras
3 ottos
and like 3 other random fish no bigger than 2"

 

[Musho3210]

probably a pH crash from the RO water. What size tank?

What are the three other random fish?

 

[toxik]

I doubt a ph crash because we only started using ro just for the past water change the first 3 died on tap water.

The other random ones are a hi fin tetra(rescued), flying fox( baby can't do any harm 1.5" max) and some shrimp.

36 gallon

 

[SJ2K]

Any chance of disease?

 

[blue acara]

Are you getting all the kissing gourami from the same shop? If all your other fish are fine its possible that the kissings are weak or diseased. Try getting them from another shop or avoiding them.