I have a 5 inch tiger oscar and he's always been healthy, but recently i noticed he doesn't beg as much and he's getting thinner. When I fed him today I saw that he was trying to each the pellets, but he couldn't keep anything down. I could see that he was trying to swallow them, but they'd just pop right out of his mouth eventually. he can't even get little beta balls down. Any suggestions? Is it a blockage of some kind? It's a cycled tank. Ammonia and Nitrite are 0 ppm. The Nitrate is around 20ppm (15 out of the tap). PH is 7.7.

If the fish had a blockage then you will probably notice a swelling in the abdomen. It isn't uncommon for fish to almost become disinterested in food and although they seem to want to feed, spit it out. However, if your fish is getting under then there may well be an underlying problem somewhere. Now since your water appears to be in good condition we can rule that out. Have you tried a completely different type of food? maybe some frozen shrimp, see if they ask you will eat this. Try this and see what happens. Apart from that and really haven't got any other suggestions. I suppose there is always the possibility that there is something more serious wrong with your fish but that is something I really couldn't tell.

Are there any tank mates? - maybe he is getting bullied

Sometimes oscars get bored of their food, have you fed a varied diet or have you fed the same pellets everyday. Try your fish on different foods, check the site out for different foods to feed to oscars