The danios are quite active little fish, other active fish would make for a lively tank if that is what you want. A small school of tiger barbs 6 or maybe a few more would be a decent choice. While they have the reputation of being aggressive (which they can be in some cases) they should fare well with the danios. Cherry barbs would work instead but try to resist getting only males which are the better colored of the two. There are plenty of tetra species that would look nice as well, again get a group so they can school. Pristella tetras, serpae tetras, red eyes, black neons, etc etc. Rasboras would also be a nice choice imo. Harlequins "red" rasboras are popular but there are others that are nice too. Brilliant rasboras, scissor tails, etc. Most of these guys tend to stay in the mid to upper levels of the tank. Some of the smaller loaches could be an option for the bottom, or a group of corydora catfish are hardy, active, and efficient "cleaners." Again they do best in a group so dont get only one. If you want to get something a little more "exotic" a pair of dwarf cichlids should make a nice display and shouldn't harm the danios, although they might chase them away from their "territory" on occasion. Think blue rams, or maybe a pair of apistogrammas. Caucautoidies or Agassizi's can add some color and attitude/interesting behavior. Wow the more i think about it there are so many cool fish that would work. While it is ultimately up to you, I would strongly advise against getting into the noah's ark approach... two platies, two mollies, two of these or two of those. The fact that you are researching this in advance tells me that you should be very successful with your tank. Oh yeah, if you decide to get a "scum sucker" I would avoid the "common" plecostomus and go for a bristlenose or even a group of otocinclus catfish instead. They do better work and wont outgrow your tank this year. Happy fish keeping.
