scubadoo
Fish Crazy
I know what'll get the cr@p off your substrate - use a syphon and keep the tank clean with maintenance !
Fiddler crabs are obviously not suited (as you've been told, they are brackish, need their own setup with 2/3 of tank being land - also even if it wasn't for that at some point your fish would probably kill the crab).
Clown loaches need to be kept in groups of at least 3 though 5+ is better, they do live a long time (I heard approx. 30 years - though it could be longer) and they need at least a 55 gallon tank (that is at least 4ft). they do not eat faeces.
In fact I don not know of an aqarium pet that does live on faeces, Though the odd bit will be eaten by a stupid fish or soem shrimps etc, they cannot live on it and they will add to the bio load rather than help with it.
I think you might need to rethink what you want and what you want it for. Hope that helps
from my research clown loaches rarely exceed 10" in home aquaria, and they are continously growing until they die.
the same is for oscars. oscars will never reach their full size because they keep growing as they age. sure all fish do, but we're talking it can still grow an inch or 2 a year still. oscars grow in spurts like plecos not all at once. they will shoot and grow like 6" then will go kind of dormant and grow like a 2" in the same spand at another point. then back up fast. its weird.
as for the fiddler crab, i got one because they eat detritus (remind you that's not "poop") it is actually the broken down decaying matter (can be poop but not necessarily poop). it is very much like the humus that earthworms eat. but anyways, they feed off of the microorganisms that grow in this detritus. they also do best if yo do feed them a special diet suited for them.
as for the brackish, i cant vouch for that as i never heard it, though i am sure it's probably true. people on here know what theyre talking about. but i will warn you. i got a decent sized one and put it into my tank. well my biggest fish is still smaller than your smnallest and mine killed my crab and ate it. i want to get aq true blue lobster, but after seeing that no way am i going ot get a $20 lobster for my fish to get a gourmet meal.
if you need something to act as a "clean up crew" i would recommend a single bristlenose pleco. they grow to about 6" if i'm not mistaken, eat algae and leftover food. if you would like some crabs, jsut set up a seperate tank for them, i think i may do that (after i get my lobster of course).
like stated no creature lives soley off of "poop", they need other means of food in their diets. the eaqsiest way to solve problems of that sort is to understock, overfilter and do routine maintanance. other than that tht problem will always exist.
well i hope that helped.