This comes up so often, I really wish there was a way to once and of all make people realize that a DIY solar system using poly pipe is NOT going to be as efficient or inexpensive as buying the cheap pool solar mats ... when you consider the heat gain in the pool.
The only way the DIY is cost effective is if the pipe is free, but even then you will not add as much heat as the panels unless you use a LOT of pipe.
FYI, I had 2500 feet of this stuff on my roof when we bought my current house that was broken out into about 8-10 parallel circuits, sure it added heat to the pool, but not as much as I wanted and I got tired of dealing with the constant leaks. So I installed 500 sqft of used solar panels myself and gave all the pipe away for some tiny amount of money. We were swimming in 80 degree water in mid-March ... without the use of a solar cover.
By far the cheapest way to warm your pool is to use a solar cover (at least at night) to retain the existing heat.