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i have two oscars an albino and a tiger the tiger is ill for the last three days he's been just sitting at the bottom the of the tank , not eating and lossing weight though he looks fine does anyone have any ideas what it could be i have a feeling that he isn't gonna last much longer

Need some more information,
Tank size
age of fish
other fish in the tank
Ammonia reading
Nitrite reading
Nitrate reading
How is the other fish acting
Water temp
how often do you change water
clean the tank.

i have had the fish for about 3 weeks
they are about 3" long
i also have 2 plecos in the tank
the tank is 55gal
as for the water i don't think it has anything to do with it cause all the other fish are acting normal

i forgot to add that i clean the filter once a week and change the water about 20% every week or so

Doing water changes every week is good but cleaning your filter every week is not so good because you end up killing all of your benificial bacteria. You should clean your filter only once a month and your water changes should be about 30% every week set yourself a schedule where you do a water change one week then a gravel vacuum the next week which means you do a water change with it, then just a water change the third week, followed by a water change and filter clean in the fourth week. This will help keep your water quality stable. Your tank might not have enough beneficial bacteria in it and your oscar might be effected by it. The bacteria is what keeps your water quality in check.

Sounds like your tank is not cycled. Get a test kit for ammonia, and check the level. Just because one fish shows symptoms, does not mean the others are fine.
You will have to do water changes, dependant upon the level of ammonia. Once the nitrifying bacteria have established for the ammonia, you will then have to watch for nitrite. It takes about 4 weeks to complete the cycle.
As for cleaing your filter, what type do you have. For Oscars you need lots of filtering. It should filter the water at a rate of 10x for hang on back types, and 5x for a canister. So you have a 55, this means your filter should be rated at 550 gph for a hob, or 270 for a can. Water quality, is the most important issue in fish keeping
Post back and let us know.

i started treating the fish for hexamita and the tiger is now eating and swimming around it seems to be getting better
thanks

WHY hexamita?? as this is a new tank and the Oscars are not OLD enough to even concider getting hexamita, you treated a fish for something he does not have..Water changes are your best bet..Get the test kits OR take a sample to your fish store and have them test the water..25% water changes are in order for any kind of spike in Ammonia, nitrite that may be high..0.2 and up..You need to do this about every 3 days to keep them down..best to test the tank your self, so get some Liquid test kits, they are more accurate..

i tested water and everything is fine and the tank is not new i have had it 5 months though these fish are new
i have had fish in it in the past but didn't live through the transport to our new residence
i have been treating them for internal parasites because this is the only thing i can think of that might be wrong because of the fishes symptoms

Hexamita actully can reside in a healthy fish in small amounts, its just the intial shock of being moved or another stress of somekind that they start multpblying due to a weakened immune system.

Your initail description did however lead people to other conclusions which is why the more info on a tank in detail etc when posting here results in better help.