I love serpae tetra's, one of my favorite fish, bright red, purple/orange sheen, and if kept in correct #'s, they are not overly aggressive. I also have kept Mbuna and have a 125 currently set up w/Tangy cichlids. If you go with cichlids, you prob should rehome all your fish with the exception of the Danio's, they can be kept as dithers if you keep bottom dwelling Tangy's. pH is good above 7.5, what is yours? As far as cichlids being hard to keep, that's really not true. They are no harder than any other fish to keep successfully, as with any fish, their needs need to be met. Substrate is really dependant on fish and tank type, I like soil base, Black Flourite cap, for planted tanks, but others like sand. For my Tangy tank, I went PFS, it's perfect for the fish I keep. First decide what fish your going to keep, then decide the substrate. Hope this helped.