i made a cheap solar heater with a submerible pump and garden hose.

the pump send water through the hose, which is layed out on the ground.
sun and the ambient air termpartures heat the hose which heats the
water. i was able to bring the temperature of my 2000g pond from 7
celcium to 17 celcius in about a week. i only ran the pump during the
daytime when the sun was on it. the nights in ottawa canada are still
really cold and the solar heater would just work in reverse during the
night. infact, i only ran it when the sun was out and hitting the hose.
this was about 11am to 7pm.

keep in mind the last few days have been unsually warm. temperatures in
the day were usually around 20+ celcius.

it was very cold today so i didn't run the pump. it also rained. air
temperatures were around 6-7 celcius. the pond water was around 11-12
celcius. it took days to heat it up but only 12 hours of cold
temperatures and rain to cool it down.

hope this helps.