Electrical doodad's Question

[daplatapus]

Because electrical is probably my weakest field of knowledge (go figure with all I'm trying to build lately) can someone help me determine where I would want a mechanical relay as opposed to a Solid State relay?

So say I have my heaters, Ca reactor, main return pump, skimmer, ATO, sump lighting, and whatever else you can think off of the top of your head.