low maintenance tanganykan cichlids tank

Hi,

I just add to my tank a 5 inch female fiddler crab(both claws same
size)

I started to combine red crabs and fiddler crabs in my tanganykan
cichlids
tank, but the pet shops always caution me that the crabs might eat my
fish.
Most pet shops only carry 1 inch red crabs or fiddler crabs. I did
research
on the web, and most crabs eat fish if they can catch, but if not
disturbed,
they will not pursue a fish target. Mostly eat particles glued to the
gravel.
Luckily I found a 5 inch one is a small fish shop. It is awesome,
seeing a
crab behavior without using a magnifier.

I still keep my tank running at very low maintenance, no water changes
needed.
Feed only once a month.

The same fish shop, also carry the biggest non salt water eel I have
ever seen.
the head size was about .5 inch diameter and was very long. Anyone
knows
about if eels are agressive? I plan to add to my tanganykan cichlid.

My tanganykan ciclids consists of 3 baby tetracephalos, 2 baby calvus,
1 adult very big 3 inches bright orange leleupi. The leleupi is even
more aggressive than the 5 inch crab.

cheers