well actually you guys have it pretty well answered but I'll add to your comments. First off I would like to say that this is actually quite simple but I understand these fish and stingrays are quite expensive and important to you guys. I had a girlfriend who works with these guys at west edmonton mall and lots ended up getting sick and died apparently because some pipe above was dripping into the tank. Getting started you would need to make a concrete pad and I'm guessing probably 4" thick, with 1.5" thick styrofoam underneath and place 1/2" pex plastic tubing about 6"-8" apart. For the heating source I would suggest a combo water tank, this would heat your water for your house and would do the heating in the floor. I use a john wood combo(55% eff. and about $1100) and a polaris combo(93% eff. and about $3000). I use this same system for small houses in the basements but use 12" centers for the tubing. You would also need a pump with controls and timers because its code here because the water in the tubing could get stagnant(about $550). instead of a thermostat you would use an aquastat. I'm not sure about the rays liking the warmer bottom but I do know the concrete is not hot(unless you put a floor sensor), it is just warm and takes the cold off the concrete. it is also very even heat and constant so there is no change in temp. is this a plastic tank or are you making it all concrete? you could put tubing in the walls(I've done it on tanks used for plasma cutters so the tanks wont freeze in the winter but they never had any tubing underneath) but I don't think its necessary. any more questions?