hi guys,just wondering if u's can helpout,,every 2nd or 3rd time i do a water change i get a tank full of litle white worms,iv put in worm tablets a few times now but they keep coming back,,also the oscar gets real slow and spends most of the time on the bottom of the tank,,cheers

How big are these little white worms? There is a possibility they are planeria which are free living parasites that can live in your tank. They can often be introduced on live plants, or in food. The rule of thumb is that if they are not attached to your fish, they shouldn't cause too much of a problem. People just don't light on because they can be unsightly. They can be quite difficult to get rid of. Normally the only reliable way of eradicating these horrible little critters is to completely strip the tank down including filtration. Obviously this is quite drastic as you would probably have to recycle it all over again. I think you may have to manually remove them. Never had them myself, maybe someone who has good you better advice on how they got rid of their problem.

Have you tested the water? Water problems could cause fish to become lethargic. However, if they are doing it directly after water changes, it could just be sulking behaviour. Give us some more information on what is happening

Had the same thought about planaria reading that, or there are as well some smaller kind of free living worms (nematodes) that are common in fish tanks. I'd consider both kinds harmless when it comes to doing harm to fish, they might be an indicator though that there is too much waste in the tank or substrate if they appear in really big numbers.
Being careful not do overfeed and doing regular thorough gravel cleaning should keep them in check, else there are fish that love to eat such critters - think macropodes for example if I remember correctly - but will neither get all of them. And that isn't really practicable neither. The best method is certainly what Penn said, but yes, who wants that.

Water parameters would be nice to know as far as nitrates etc. go, and I'd personally be careful with adding chemical remedies, especially repeatedly ... dunno what's in those you use, but some that are meant for snails and such contain copper or similar stuff that isn't healthy for fish neither. Copper in particular is famous for causing sudden deaths in plecos a long time after treatment.

Hope there is nothing wrong with your Oscar, and keep us posted.