I have a 75 gal tank with one Tiger Oscar, one Jack Dempsy, one Pleco, and one Clown Loach. I set up the tank about 4-5 months ago and since the beginning, these are the only fish I have had in the tank. When I bought the Oscar and the Jack Dempsy, they were both around 2-1/5 to 3 inches. Now the Oscar is about twice the size of the Dempsy, but lately the Dempsy is constantly fighting with the Oscar. (Neither the Oscar or the Dempsy bother with the Clown Loach and the Pleco.) I'm really worried about the fighting between the Oscar and the Dempsy. The Oscar seems to get pretty beat up. The Dempsy will go at the Oscar head-to-head and will bite him on the lower lip and not let go. I'm not sure what to do at this point. Help!

The Jack Dempsey is one of those fish that will either work, or it won't. Somewhere on this site I do warn about problems with aggression when keeping Jack Dempsey's and Oscars together. You often hear people saying that they don't have a problem. Obviously you are one of the people who are unfortunate enough to have a problem with keeping these two fish together. I can't see anywhere around it apart from splitting them up. Your tank is not big enough for a divider so I should think about finding a new home for one or the other.

Dempseys have a reputation for aggression(they are named after a boxer), however this is not always the case, like oscars each one is different. I have one with my oscars and he never gives them any grief.

If your oscar is getting injured then you should separate them, if he is just losing the mouth battle but not getting injured then leave them and see how it goes

Clown loach should not be kept on their own. The are social fish and need to be kept in at least 2 or 3's. When i kept community fish i bought 2 clown loaches but one died. The one left used to hide away to the point that many a time i thought it had died. As soon as i put a couple more in, all three were about all the time

my dempsey is really placid and is great with my two oscars, he`s about 8-9 inches now, when i bought him he was alot smaller than the oscars almost the size of a snack! the first month or so he just hid and the oscars showed him who`s tank it was, maybe this almost taught him respect? when i feed the oscars prawn he waits till they`ve finished before he eats! he was no trouble when i added my geophagus brasiliensis five months ago.

I am having almost the exact opposite thing happen size wise but the same temperment wise. I got my JD and Oscar at about the same time. The JD was 2" and the Oscar was 3". The Oscar ran the tank. Now the JD is 5" and the Oscar is 4". The JD rules w/ a iron fin. He is the piggiest fish I have ever seen. He eats anything he can get his hands on. He even eats the food the Oscar spits out his gills and mouth. I once saw him eat 5 out of 7 pellets I put into the tank. I thought he was going to explode . I think feeding these guys is the funnest part of having them. The other day my oscar jumped out of the water like 3-4" and took the food from my fingers as I was putting it in.

I am hoping that I dont have to get rid of one. Right now they are doing ok, as the Oscar almost always backs down and swims to "his" section of the tank. But I sure will have to keep a close eye on them.