[Zapins]

You could just buy a 20 foot coil of the metal tubing and keep winding the metal around the ballast until you have a nice coil with no gaps. That way you wouldn't have to flatten the tubing.

Also, I would be careful using copper tubing since copper is toxic to aquatic organisms, particularly shrimp, snails, invertebrates and some plants.

If you could find bronze tubing that would be better. Otherwise just use a thicker silicone tubing (sold at home depot in 20 feet roles). The heat transfer won't be as good, but it will be good enough especially if you make a nice thick coil around the ballast (overlapping several times) the heat should all be captured. Then just run water through it.

 

[Zapins]

Not to throw a spanner in the works, but assuming you do manage to get the heat transfer problem ironed out, how do you plan to regulate the temperature of the tank water? If the lights are on for 8-10 hours a day and they run at 60 C or so, even if the heat transfer suffers a 50% loss of heat you would still heat the tank water to 30 C, which would kill everything in the tank.

Even if this problem is solved, how are you going to heat the tank for the other half of the day when the lights are off? If the temperature is constantly swinging each day and night it could cause fish to stress and get ich.

I think this project might be difficult to get to work without addressing these problems. On the other hand, a simple submersible heater for 30$ will last you forever and only works for about 15 minutes out of an hour (6 hours a day max), which will probably be cheaper in the long run.

 

[Zapins]

Keep in mind that the larger the body of water is the slower it loses heat, so it would be more efficient to heat a large tank than a small one because heat loss from the surface + evaporation is reduced the larger the tank gets (surface area to volume property).

 

[Zapins]

Yes, my current idea is to simply put the ballasts on the bottom of the tank. Luckily on my stand, you can see the top of the tank from underneath!! This should be very simply.

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Sounds like a good idea. Since heat rises and cabinets tend to be rather air-tight, I think this is probably the best/easiest method of using ballast heat.

I am interested in the solenoid you planned on using. Do you have a link to the page? I think it would be useful in an automatic water change system.