I also am new to this and have been doing some surfing online to try and find some heaters. I'm wanting to build a vertical freshwater (no epsom salt) tank and so the system of heating that I will use will probably be different. As far as submerged (so nearly 100% efficient) aquarium heaters go, I have found that it is recommended to choose a heater wattage that is determined by taking the number of gallons you are heating and multiplying that by 5. I am wanting to try and heat (from the dimensions I am expecting to use right now) 200 gallons, and according to that guideline I would need (at least) 1000 watts of heating capability. I'd probably play it safe and go higher (I personally don't like to do things right at the threshold) and so maybe 1200W in my case, but I plan to zone the heat - rather than having one heater/heating element I plan to use 2 or maybe 3, each with their own thermostatic switching. Maybe 3 of these could do on a freshwater system
http://www.aquarium-supply.biz/Hydor...p/rhy00320.htm
The thing to keep in mind here is that 5 watts for every gallon guideline that I read (somewhere on an aquarium website) was given with respect to a submerged heater (one which will impart almost all of its heat into the water with 100% efficiency), and also for aquariums which would be kept not so much higher than normal room temperature unlike float tanks which are kept far above room temp. For a floatation tank where one intends to use a less efficient heating system (such as putting heating elements under the vinyl liner having some of the heat going into the plywood etc?) I am guessing one must use even higher wattage/gallon. Otherwise you run the risk of not being able to ever reach and keep the temperature at the level needed (~93 degrees or so) If this is not correct I would ask that someone correct me on that.
The issues of which I have read concerning epsom salt water are:
It is important not to use a heating system which will put a metal (such as copper, iron, titanium, etc) into contact with the corrosive salty water (this is also true for metal-contruction pumps) or you will end up floating in a system which has those metals at potentially toxic levels which the skin can/will absorb in the hours and hours of floating time. You can read more about this in John Lilly's "The Deep Self". It would seem like you can however keep the heating elements separate from the water by having them under a liner (if you use one) like many/most do. Perhaps there are some ceramic-coated heating elements which would work okay for submerging into the water.
*My* own idea (for what it's worth) for how to design a horizontal plywood-frame tank would be to double-up on the plywood thickness to allow for the 'inner surface' pieces of plywood to have some routered-out channels in which the heating elements could be placed. Then over the plywood (inner facing surface) put aluminum sheet metal on the bottom and side edges (at least as high as the water is intended to sit). Then line the tank with the vinyl as normal, of course making sure to cover the aluminum COMPLETELY so as not to allow the aluminum to make contact with the epsom salt water (directly or in the air in the tank). In this fashion, one could utilize the the thermal conductivity of aluminum to spread the heat out across the bottom and side surfaces rather than just having the heating system right unde the vinyl and thereby focused in 'strips' or right around the coiled resistive heating elements or what-have-you. Aluminum sheet metal is pretty inexpensive and any nearby metal supplier can get it for you (and cut it for you if you don't have a suitable saw or sheer, etc. If this were all done well, it might be possible to make it so one can later remove and replace the heating elements (when they eventually wear out) without dismantling (and maybe without even draining) the tank! Let's hear what other have to say about this.
Cheers.