clam advise

[sergifed91]

I have a 75 gallon. going to upgrade here soon to a 90 gallon. not going any bigger with limited space. but been thinking about a clam. not sure if there would be enough room to add since every thing i read they get up to 12 inches in the wild. are there any that doesn't get 12" that would go well with my tank?

 

[OrionN]

Tank size and clam isn't very important. What important is if you can provide the nutrient and calcium needed for some of the large clam species.
Different clams species have different Max size that clam will "never" reach. Get a clam to 12 inches take some time, even the fastest growing species like Gigas, from 2 inches it will be 4-5 years of optimal grow to get to 12 inches.
The smaller species, it is the light that is important. They all needed high light. There are 7 species of commonly available clams for aquarium and a handful of exotic that cost thousands. Look at Crocea, Maxima, Noae, Squamosa. All will be fine with your tank in term of size. Most colorful of which are Crocea. Picture below are just a few example of possible Crocea (three maller ones on top) and Maxima (larger on the bottom)colors.