The DIY coiled heaters are difficult, you can't just plumb your return through 100' of 1/4" pipe the resistance is too great. You either need some kind of small auxiliary pump that's appropriate for the high pressure low flow application or you need to take a Tee off a higher pressure section of return, straight off the filter output, and then return that later in the return line or direct to the pool - so that the pressure differential will generate low flow through the heater without restricting the pump.