This has probaly been discused before.If so please point me to it.If not,do these things really work?Pro's.Con's.They claim to raise temp 4 to 8 degrees and doesen't effect the water chemestry!

I've never read a positive review about these things - snake oil is all it is.

Even though the solar fish could work in theory they don't seem to work at all well in practice. They form a protective layer on top of the water, but even a very slight breeze can disrupt the protective layer. A solar cover is far more effective.

I found out what is actually in the HeatSavr liquid in the fish! We know that it contains isopropyl alcohol from the MSDS and another ingredient that did not have to be listed because it was non toxic. The other ingredient is just paraffin!
IF the surface of the water is perfectly still then this thin layer of wax might keep a few degrees of heat in the water overnight but if there is any surface disturbance from wind (or if any pumps are going) it is ineffective. At best it seems to keep in about 2 degrees from what most people have reported and at worst it does nothing.

I have no personal experience, buy my pool installer had a customer that used one of those oil-based covers (solar pill, liquid fish) and said that it did not do much and that he felt like he had been swimming in crisco every time he got out of the pool. Yuck.