I always liked these guys... I was offered three of this guys for free. I was wondering if I should take them. But I would need some info on them first:
1.) What will/would they eat??
2.) What size will the grow to??
3.) Should they be ok in my 20 gallon long??
Does anyone have pics???
Should I get them?
-Alex
Well I'll start by answering your questions. Freshwater clams, are similar/identical to their salt water cousins in terms of diet. Clams are filter-feeders, which means that freshwater clams will most likely filtrate what's in your tank water and eat the micro-organisms and small cultures that are hopefully in your established tank. I personally found freshwater clams to be very amusing (I used to have them in my tanks in Hawaii) as they will "move around" your tank and it's really neat to see them open their little clam shells to filter in and out. What size will they get to? Well that would depend on the species that you acquire. However, I think the biggest a freshwater clam will get (if they will even live that long) is probably the size of a quarter (or 1 Euro if you live in the EU). I think they will definitely be okay in a 20 gallon. However, may I ask what other tank-mates these clams will be housed with? Anything aggressive or bossy that would attack the clammies? And pics, I would google image. But here's one (not mine).
erm, i personally think keeping a clam is a pretty bad idea
they filter the water, and are primarilyherbivores, which means you need to feed green water, i think that while space wise, it is fine, it will be hard to feed. even in saltwater tanks which generally have much more micro organism life they need to be fed purposely through phytoplankton cultures or ready made stuff.
for the most part, clams have no part in freshwater tanks. a pond maybe, a very large very green tank? might last a while, but eventually it will run out of food
Also note, some of the young of the freshwater variety are parasites and will go underneath fish and look like ich or other parasites. It's pretty stressful for the fish and owner.
I used to keep clams... no longer. It's difficult to keep them fed and when they die they can release toxins into your tanks. Like others noted, unless you have really cruddy water, they'll slowly starve. From what I've read, the larvae are very species-specific, but it's still not worth the trouble. Get a bamboo or vampire shrimp if you want a filter feeder They can forage if there's not enough good stuff in the water. Clams can't.
Unless you've got a planktonic algae culture mussels are really hard to keep. Also, in the US and Canada freshwater mussels are THE most endangered group of species. That means you need to be really careful where you get them from. These guys are slow growing and slow to reproduce. They use a fish host to attach their parasitic larvae (known as glochidia). The glochidia hitch a ride for a couple months on the fish before dropping off. This rarely harms the fish. If you show a picture of the mussel I might be able to ID it for you.
What size will they get to? Well that would depend on the species that you acquire. However, I think the biggest a freshwater clam will get (if they will even live that long) is probably the size of a quarter (or 1 Euro if you live in the EU). .
Some get quiet a bit larger than a quarter, Just walking along the shore line at some US lakes,I have seen shells that were 4-6 inches laying about . Also I agree with DoctaQ
Freshwater clams, are similar/identical to their salt water cousins in terms of diet. Clams are filter-feeders, which means that freshwater clams will most likely filtrate what's in your tank water and eat the micro-organisms and small cultures that are hopefully in your established tank. I personally found freshwater clams to be very amusing (I used to have them in my tanks in Hawaii) as they will "move around" your tank and it's really neat to see them open their little clam shells to filter in and out. What size will they get to? Well that would depend on the species that you acquire. However, I think the biggest a freshwater clam will get (if they will even live that long) is probably the size of a quarter (or 1 Euro if you live in the EU). I think they will definitely be okay in a 20 gallon. However, may I ask what other tank-mates these clams will be housed with? Anything aggressive or bossy that would attack the clammies? And pics, I would google image. But here's one (not mine). 
