There have been a few suggestions mentioned since my last post, so will try to cover all.
First, Diamond Tetras...not with Congos. I have had both, and spawned both many times or more correctly had them spawn prolifically on their own, and both are stunning fish, but on their own (too much alike). When it comes to a shoaling species [some call it schooling, but technically it is more correct to use shoaling with most freshwater fish like the characins, cyprinids, cories, etc.] something as "outstanding" as either of these fish is much better separated. A large shoal (group) of either Diamond Tetra (species is Moenkhausia pittieri by the way) or Congo Tetra (Phenacogrammus interruptus, one of ten recognized species in this African characin genus and the one almost always seen though one of the others which is similar may be imported rarely). I can provide details of either if asked.
Second, continuing now with the Congo Tetra as primary upper fish. If the Congo Tetra is to be "it" for the upper fish in this long a tank, get a larger group than 10-12, more like 20-25, again a rough male/female split. Floating plants are essential as this fish does not like bright overhead light, and in any event the incredible metallic-type sheen will only be striking under diffused light. Some open areas but about half of the surface covered not thickly necessarily but with substantial floaters. Every time I thinned out my floaters in their tank, they moved down in the water column for days afterwards, until the cover grew back to their liking. They will swim and remain mid-level and slightly above, always. By "substantial floaters," I mean floating plants like Water Sprite, Water Luttuce, Frogbit, and some of the stem plants like Pennywort which grow nicely left to float. Tiny plants like Salvinia and Duckweed do not work here, and in fact are more of a nuisance than benefit light-wise.
As for cory numbers, again if just Congo T upper and cory lower, at least double if not triple. In this tank I would have 35-50 cories, no fewer, with or without the pleco or similar loricariid--a common whiptail in the Rineloricaria genus, one, two or three, would be my addition with the cories, as this fish is prehistoric looking and thus interesting, and very harmless and does not attain more than 4-5 or maybe 6 inches but is so thin this is hardly noticeable. Sand is crucial for cories, that has been mentioned. I use Quikrete Play Sand which is available in NA, there is a similar in the UK ("Argo" perhaps? UK members can confirm), and these are totally safe. Not all play sand is, and no other industrial sand is absolutely "safe,"--though this is likely to start another sand argument, but fact is fact and that is the bottom line, and beyond that it is opinion and risk. The only better sand is pure river sand, which may be available somewhere, not sure where.
I would tend to recommend one, two or three additional smaller upper shoaling fish species. Even with the above numbers. Surface fish for example would be a wise addition to this tank, first because of calming the Congo and cory below (believe me, this does make a difference), and also for surface interest. Hatchetfish of the Carnegiella genus only are ideal, if say half the surface is open and half plant, this is beautiful. I can detail species if asked.
Cyprinids (barbs, danios, rasboras) are options, but much more limited. Will leave that topic unless asked. Characins for additional upper fish, with the exception of the Trigonstigma rasbora species, are better.
Water parameters (heat specifically) were mentioned. GH on the soft side (say up to 12-15d GH), pH on the acidic side (depends upon the GH and KH but will likely be preferably below 7 but up to mid-7 is OK here), and temperature in the low/mid range for either tetra will be OK for the cories, all species. Range for Congo is 23-27C/73-80F so low/mid means around 76F/25.5C would be my upper limit. I spawned all these fish in tanks that remained at that temp for years. GH/KH was zero (my luck with our source water!) and pH so low I can't measure it. But these are the absolute ideal parameters, increasing GH/pH is not prohibitive so long as it is not on the extremely hard side.
