I use the Turbo Solar fish and it works for us. I believe a bubble cover would work better, but we have a large odd-shaped pool and not much deck space inside the safety fence to store the rolled cover, even in pieces. My kids are another reason to avoid the bubble cover--a friend's toddler almost died when the pool gate was left ajar at a party and he went into the pool, slipped under the cover and wound up at the bottom before being found just in time.

We have a large temperature drop in the air in N. California (30-40 degrees sometimes). The fish keeps temperature loss in the pool to 5-7 degrees, sometimes less depending on cloud cover.

Sometimes the fish don't work and I can tell right away by the pool temps. The company that distribute the fish (sunsolar) is really good and will send replacements for bad fish if you send the proof of purchase, so keep the receipts. They don't have this on the instructions, but if you have the turbo they told me to hold the turbo at the base when you first put it in the water (not held upside down, just put your hand onto the bottom of the fish). Within 4-8 seconds, you should feel the vibration of the pump activating. If not, remove it from the water, give a small whack against the pool deck and try again.