Oscar Troubles

Oscars can be quite the bullys, and they also can pout very well. I have 2 in a 100 gal. tank, and one always picks a fight and gets his butt kicked! If he gets whooped pretty good, he will lay at the bottom and pout for a couple of days and then they are friends again. I think your other oscars are ganging up on him or someone is being a bully. If he continues to lay at the bottom for more than a couple of days, you might want to consider putting him in another tank.

oscars

I have 5 oscars, 2 albinos and 3 tiger, They are fine, I have 1 moody one, but it soon comes round when it gets a little attention.
They are fine with the other fish in the tank and only try to eat the pleccy occassionally.
My oscars are very hand tame and have a varied diet of fresh and dried food. in fact they'll eat almost anything, I've found that really they own me not the other way round. They are such a pleasure to keep and are much like a dog, very rewarding. They will attack a stranger so people are adivise not to put their face to close or their hand in the tank.
I've never really had a problem of them shocking, only when they really dislike something; maybe a stranger or to much activity near the tank.
I never turn the light from complete darkness to light or vice versa as that shocks me let alone a fish!!!!
there is always some form of subdued background light.
They are in a huge well filtered tank, i also due a weekly 25% water change,
such dirty messy fish...... a small price to pay for the enjoyment that I get from them.

It sounds like your tank is very heavily stocked - four oscars plus other fish in 135g is a lot IMO. Cichlids are territorial, and can fight over space, food etc. If they've established a pecking order and several oscars are picking on this one fish, it's probably best to remove him - he's got nowhere to escape, and they'll likely continue to bully him. I'm guessing he's losing colour due to stress. I'd remove him to another tank (or take him back to the store).

I never had oscars but I see them in the lfs and they sometimes rest on the bottom and I guess they are just lazy sometimes but yeah he could be being picked on.

I would also guess that its a territory thing. My oscar is alone in a 90 gal because he decided he didn't want to share it. Every other fish that went in there he harrassed/beat up until he killed them or I took them back out. I've heard many stories on here about how some oscars are just extremely territorial, so if one those gets mixed with one that isn't he's just going to get bullied endlessly, i'd imagine.

First off, if you have 4 full grown Os in a 135g tank then you're WAY over stocked and part of his problem could be poor water conditions due to that many big fish in a small space. So you know, 4 full grown Os whould be in a MINIMUM 200g tank ... 50 g for each O. Next, my Albino O does the same thing when the water starts to go south. If I don't keep up on the weekly water changes. He also gets like that when he's frightened (yours could, and probably is, getting bullied) or depressed. If your O isn't swimming around his tank then something is wrong and if he's not showing any signs of disease then he's either getting bullied in the small space or your water is heading south with that many fish and he's stressing.

My recommendations would be either get a larger tank (min. 200g) or you could get him his own tank (at least 50g) and see if that helps. Remember there's no such thing as too much filtration for Os so the better the filter, the better your fish will feel. You could also try taking the lone O to your LFS and see if they'll take it. You could keep checking back with them on him and see if he acts differently there. If so, then you'll know it was either the water you were keeping him in or his tankmates making him feel bad and causing him to sit at the bottom.

Anything less than a 50% weekly water change in a tank with that many oscars just won't cut it. I would agree that poor water quality could easily affect a fish's desire to eat, swim or interact as usual.

With oscars and other big fish, water quality goes south really fast and just because ti is clear doesn't mean it is healthy.