Sick male beta fish?

Question
hi, i have just bought a male beta fish and introduced it along with a few tetras and a floor feeding fish, we had the water tested prior to buying these fish just to be sure and it all came back fine, my male beta seems to be very subdued and lyes on the bottom of the tank alot, when food is put in he seems to go for the food and eat but then returns to the bottom again and appears to lie down. all the othere fish seem fine and full of life. i am new to beta fish and am not sure of there usuall behaviour. i am not sure whether to take him back to the shop as we have only had him a few days now? his behaviour just dosnt seem quite right to me. joe

Answer
Hi,
This is an extremely difficult question to answer without more information.
Do 20 percent water changes every 1 or 2 days for a couple of weeks, and treat the tank with Melafix, to kill any residing bacteria/fungus.
When you say "fine" do you know the exact readings for the water? I recommend you buy your own test kits for ammonia, nitrite and nitrate, and pH if possible. The ammonia and nitrite need to be 0.0 permanently, and the only thing that can lower it is frequent water changes. The nitrate is safe under 20ppm. Live plants will help lower it. Bettas prefer a pH of between 6.5 and 7.5.
Move the filter up so it is making bigger ripples in the surface of the water. This is help oxygenate the tank.

The fish is probably just suffering from stress or pH/nitrate shock, and it should subside after about a week, but there are many other things it could be, so without a good picture of him I cannot be 100 sure.

If you have any more questions, please ask.
Best of luck,
Kathryn