2 x Rena XP4 and you would be good with filtration on a 75g tank. As I mentioned in the PM you sent to me, you cannot over filter african cichlids, they need it as they are messy.
15-20 fish in the correct ratios is very reasonable, sadly though what is not reasonable is your stock
1 yellow lab - need to be in a 1 male to 4 female ratio otherwise they would pick on each other, single fish are not really ok as they can get stressed
2Demasoni - BIG flag on these unfortunately. First they are very aggressive and if you have 2 males, once they mature to adult size they will kill each other. They need to be i male to 7-8 females, that way the male has multiple females to harass once in breeding mode.
1 Ice Blue Zebra peacocks - Any peacock is not a good choice to house with mbuna *the other fish above), peacocks compared to mbuna are very sedate and will get stressed easily.
1 Ruby Crystal peacock - same reason as above
If it is possible to take the fish back and exchange them, perhaps look into more yellow labs, pseudotropheus acei (yellow tail), cynotilapia afra, rusty and red zebra *red zebra will cross breed with the labs, so you wouldnt want to sell any of the fry).
What age, length are the demansoni, you might be able to keep them for a bit if they are very young.
Eel is not a good choice at all, mbuna will not hesitate to attack it and kill it.
As I mentioned, Bristlenose plecos, synodontis Multipunctus are good bottom feeders and cleaners, bare in mind though they need a piece of driftwood to keep them happy.
If you want to go the sump route, look at doing a wet dry filter rather than a full sump, wet dry can be made very easily with just a few bucks on boxes from walmart and other stores.
Benefit of having a wet dry is the surface area for good bacteria is enormous compared to conventional filters even canisters.
Did you cycle your tank also? If you are unsure as to what cycling is, you can read up here