What is the best filtration system for a red claw crab?

Question

I know they are crafty and will get out of the tank at any given chance.

I don't really want to have to use an undergravel system because the aquarium is already set up. Just doing frequent water changes now.

Answer 1

I would use a submersible filter like one of the Fluval plus models (which you use will depend on the tank size/water volume). A glass cover with a notch to allow the power cord out will keep your crabs where they belong.

The crabs need a place to get out of the water, so keeping a low water level will strain the motor of a hang-on power filter. Keeping the water level at the rim with a dry area is asking them to get out of the tank (unless you set up an wood or rock "island" at the center), but if this was too high out of the water, this wouldn't allow a cover (and if there's another way for them to get out, they'll find it).

They also should have some salt in their water.

Answer 2

When keeping semi aquatic critters like newts and frogs. I usually don't add a filter in the tank because I don't put much water in it. The deepest part is only about 3" and I usually have a sloping bank where they can climb out to tan/dry. I also have a log in the water if they decide to hang out above the water.

Once every 2 weeks or in some cases one depending on the waste produce, I drain the whole tank of water and replace totally with fresh water. For this exercise, I hose down the gravel that forms the bank too, to get rid of waste that have set into the gravel. After I reset everything, it's as good as new.

You could do the same with your crab setup, I just think you might want to add some dechlorinator since your crab breath through it's gills under the water.