yup, remove the heater.. copper will not be good with the fish, but the spa will make a nice tank..
use the ibc cut in half for deep growbeds, i've seen IBC 's cut lengthwise as well..
i've read up on a few of the "kit" sellers in the US, and the systems i've seen would work, but not anywhere near the volume of fish and plants they promote.. and the biggest failure point i've seen on systems has been from overstocking...
but i'd really like to get some "nice" growbeds (i like byap's gb's)