Crusty has pointed you in the right direction for fish etc. There are also three more places in Phuket that I can "find", one other place I saw before but must have been down that magic road I can't find again.
My old man got a really nice set up and he's very happy. Unfortunately he seems to have a similar problem as Bee does and the fish "apparently" turn belly up.
Head out of Patong, over the hill toward Phuket. When you get down the hill and are at the traffic lights, turn right. Immediately on the left (5 meters) after turning there is a fish shop (only open a couple hours each day).
Continue driving down the same road past the fish shop (for the second shop) and after around 4 minutes (you've driven past bungy jump) about 500 meters you'll find a small shop on your left just before you get to the busy area of shops after the bend and before the golf course area. It's a small mum and dad shop but very good, good prices and also they will take care of your delivery and set up. Just make sure they wash the gravel before they put it in your system (or the system they supply) as it was VERY dirty when they did it for us and I had to redo it all.
The last place is on Choafa Road in Phuket town. Opposite the school at the intersection and traffic lights. Not sure of the exact address but it's not far down from Phuket Art and Sign. They are farang owned, around 4 to 5 times more in cost, their plants die..... but hey, they've still got stuff
Good luck to you!