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I don't have any experience but have been reading on this site about them and the biggest concern is that you really can't tell when they die because they burrow. That's something to watch for.
I am glad you posted this because my LFS has Shark tooth fresh water clams. They have a very large shark tooth shaped extension on the back of their shell. Really cool but I wanted to know about keeping them before investing in the them. That and the clams are about 4-5 invhes long so I didn't think they would be a good addition to a 10 gallon.
all i know is they will help keep your nitrate down and if you put any copper in tank for plants you have to take them out until copper is out of tank.
Sorry I have not chimed in first. www.tricker.com sells them for a bit less than what you paid I think. That is where I got mine last year.
I could tell mine was alive because about once a week he would tear a path of destruction across my tank. By destruction I mean sand trench and moved some partially burried rocks. I could tell he died when he started to open not under his own power.