my dad has loads of shells like cockels and clams can i put them in the tank if not why?

its probably not a good idea to have them in the tank, because they will slowly break down in the water and bring up the pH.

ieven use reefs from the beach i boil them if they decompose in the pot dont use em if not theyll be clean

I've got a couple small hunks of coral and some other type of brackish shell in my tank. Everything seems to be fine with them except algea seems to like growing on them.

I am not near a beach but I pick up little bags of shells at the Dollar Store sometimes. I rinse them real good! I have never had a problem with them. They look good in an aquarium! I would love to have a big ole Conk(?) Shell!!!

In a book I have called You & Your Aquarium by Dick Mills it says sea shells should only be used in marine tanks and not freshwater tanks. It doesn't say why, it just has a chart of what you can and can't use.

Isn't it b/c the shells give off calcium or something? I'd look into this more b4 adding them just in case.

The shells break down over time and will raise the ph of your tank (not always a bad thing, depends on what it was before) and yes they contain calcium. They're used sometimes in snail tanks to improve the snails' shells.