Several points have been mentioned in the latest posts. I'll start with filtration.
Filtration should be geared to the intended fish. Flow is the primary issue with this, obviously, but it should also be said that the idea of filtration having to be strong to ensure this or that turnover of water is another of those myths in this hobby. Won't go into all thatdetail, but in a 10g I would suggest a simple sponge filter is more than adequate. I have a single sponge Elite (made by Hagen) in my 10g, and a dual sponge Elite in both my 20g and 29g tanks. Now, I have quiet fish in all these, and plants.
On the Betta, I consider this fish a stand alone, meaning, no other fish in the tank. Betta are simply not community fish. While some have luck (or say they have) with other fish, this is only asking for trouble. Individual fish can differ from the expected norm in many species, but one should not rely on this occurring, but rather assume the norm. Betta often take a strong dislike to other fish, whether due to colour or whatever. But in reverse, many small fish that are otherwise peaceful will get quite nippy when placed in a small space with such a temptation as a long-fin Betta. Either way, the targeted fish will be severely stressed, leading to poor health and "unhappy" fish, and that should not be our goal as aquarists.
To the 20g, with 8 black neons, 3 glowlights, and a bristlenose. I would not hesitate to add another 4 glowlights. This should cause no problems. Be regular (once a week) with a 50% water change. If you can get floating plants to grow, all the better. A nice authentic aquascape for all these fish would be sand (or fine gravel), chunks of wood, branches, dried leaves (oak, maple, collected in the autumn from a woods or forest away from possible pollution), and floating plants. Floating plants are the easiest to grow as they are close to the light, and can take in CO2 from the air which is four times faster than from water. Floating plants for these reasons tend to be fast growing, and that means they are taking up more nutrients including ammonia/ammonium. By the way, you may know it, but bristlenose must have real wood in the tank to graze; while they do not assimilate nutrients from the wood, they do need it for the proper functioning of their intestines.
To the 10g. All corys will be better with sand, but the "dwarf" species demand it. Changing to sand in a 10g is a simple matter though; I use play sand, very inexpensive, smooth, and authentic (appearance of the Quikrete play sand from Home Depot or Lowe's is similar to Amazonian sands). I would not consider the common tetra/rasbora for this small a tank, but having said that, a group of 7 neons could probably work. But I would still say the Ember would be better. Aquascape could be as described above (mentioning this because it avoids plants except floating).
Panda Cory. This is one cory that does need some water current. They occur in mountain streams in Peru that are cooler and faster flowing than those where the majority of corys occur. I have a group of five pandas in with close to 40 corys in my 115g tank, and the pandas regularly love to play tag right in the flow from the canister, and it is very strong at that end of this five-foot tank. None of the other species ever do this. So I would not consider pandas for these small tanks; currents like this will wreak havoc in a small tank.
Byron.
