I use three 4' x 20' fafco panels on the roof to heat a 15' x 30' x 54" AGP.
I also use a cheap solar cover.
Figuring DIY hard plumbing and the extra (albeit small) additional cost of pumping the water for a season, I am into the solar hating for about $1200. For two panels that number may turn into $800.
Two panels is about my cut off for heat lost overnight (without a cover) vs. heat gained during the day.
Plus as previously mentioned, the recovered heat is not realized till later in the day.
Plus, plus, I won't go into how on some days, there are things like clouds and stuff in the sky.
The cover cost about $120.
Fairly simple math to see that the cover is the best investment.
Getting the cover to be a hassle free experience is the key to using it every night.
I'd say spend your research time and energy there.
But I highly recommend solar for phase two.