tank stocking

Question
heya, i have almost finished my new discus tank, but have 6 gallons of free space, if you follow the stocking rule for discus of atleast 10 gallons per fish.
  with the last 6 gallons, i would like to put in rummy nose tetras, or cardinal tetras. my question is, how many do you think is a good number? at my local aquarium store, the guy i talked to said that the discus stocking only applies to the discus, and you can put a few more small fish in there 'space', so instead of having 6 gallons free, he says i could actually count it as having about 10 gallons free. he then said that with 10 gallons, he would put in 8 of each, making a total of 16.
   the tank has had no problems in terms of disease, algae, phosphate, nitrate nitrite ammonia etc etc are all in check, it is well planted, has an internal filter, heating and aeration.
  in 10 gallons, do you think 16 small tetras is too many? the tank, being discus fish, is maintained as close to immaculate is i can make it! and the water quality is very good too. personally i think that 16 would be ok, but i would like to know your point of view aswell please! cheers, josh

Answer
Hi Josh,

I am really not an expert on discus, so upon doing an internet search, I see that the "10 gallons per discus" rule of thumb is indeed in existence. That just seems a little odd to me! I wouldn't put 3 discus in a 30 gallon tank, for example. However, if your water parameters test just as they ought to - and as you say, with discus, immaculate water is key to their thriving - then carry on!

Since cardinals and rummynoses are both used to enormous schools in the wild, rather than get 8 of each, I would say get 16 of one kind. The effect will be much more striking...however it is your tank, do as you like! I personally would choose rummynose tetras because they are warm water loving and their red noses are indications of the water quality. Bright red noses, and you know that all is well.

16 does not seem to much, it seems just right to me. In fact, the bio-load is going to be almost all discus - the tetras will barely even factor in. The only thing I would caution is that when the discus get to be adults and they get the size of dinner plates, an internal filter might not cut it as the sole source of filtration. As the bio-load increases with the mass of the fish, you might consider adding a canister filter with spray bar.

I hope that helps, take care.
Nicole