What's wrong with my gouramis?

I have two Gouramis which started acting strange a week ago. One is really spooked, he hides all the time and I haven't seen him eat in a week. His scales have gone a dark colour. If he does venture out, he races back under the log if someone even goes within 5 feet of the tank. The second Gourami is still eating fine, but hides when not eating. He doesn't seem to be too worried about my presence near the tank and he looks healthy except his body seems to be slightly bent to the right all the time. I thought maybe it might be due to using a copper sulphate treatment for snails, but I used it in really tiny doses because I've got soft water. I've also done three water changes since then and they've only gone really strange since the second water change.
Anyone any ideas what's wrong with them?

 

How old is the tank?
Are there any other fish?
What are your ammonia, nitrate, nitrite, pH levels?

 

have you run activated carbon in your filter since using that? do you have an air stone? do you have any pics? can you dI'm the lights and see if they act differently in the lower light?

does this match them: Erratic swimming, bloating or swelling in body, black patches on body or fins.
or Small dark spots on fins and body. (this sounds likely and is Black Spot, treatable with a product like parasite clear.)

 

To answer both those responses; the tank's about 4.5 months old ( not including the 6 weeks it cycled before I put fish in), it's a 54 litre tank with 12 other fish (6 neons, two corydoras, 1 betta, one molly, and two others that I can't remember what they are, but they're only small, about 0.5 inch.) The chemistry seems to be fine, 0 nitrites, 0 ammonia, and lowest levels of nitrate that my strip can record.
I changed the water about five days after using the snail treatment and then put the activated charcoal filter back in my filter. There's no air stone, I allow the filter to do the aeration as it's only a small tank. No pics as yet, but I won't be able to get a picture of the Gouramis as they keep hiding! I can't dI'm the light either as I just have a regular strip light.
There doesn't seem to be any lumps, bumps or otherwise on either fish, but I can't get a good look at the one that doesn't eat.

 

Sorry, I forgot to add that I can't see any black patches or spots on the one that's eating and there's no bloating on either. Could I treat the whole tank with a parasite treatment and see if it solves the problem?

 

I think it would be safe to treat the tank with parasite clear.. just read over and follow the box instructions to be sure. we'll be treating the platy tank with that tommorrow and I know not all of them have internal parasistes but I think the new guy that we rescued does and it's spreading.

the apI testing kit would be a high priority since the strips are notorious for inaccuracy and over time you'll spend less money testing with the apI kit. an air stone isn't super vital but we've found it to help keep our betta tanks cleaner.

try just low room lighting and no tank lighting to see how they act with low light if you can.

hikarI freeze-dried daphnia as well as soaking their food in a little garlic juice could prompt more appetite.

 

Thanks a lot for the advice. I'll give it a go!