The problem with tinting the inside piece of glass is that it keeps the heat between the panes and will damage the seal. You can buy windows that are tinted but they are tinted on the inside of the out side piece of glass forcing the heat out.
That is a much better idea and the method that I use.
Tinting your windows at your home will void any warranty that may apply. If the windows are more than 10 years old you probably don't have a warranty to worry about
GFCI plugs will get weaker over time. I had a whole batch that were bad that spread over 2-3 houses that I was building. I had to change all of them out. Also there are 15amp and 20 amp plugs if you have a 15 change it out.
Something else to think about is that chaeto likes 16-18hrs of light from a 5000k-5500k bulb. It might help to get some more growth and nutrient export. Just a thought
Welcome to the addiction. +1 on go slow, nothing in this hobby happens fast. Their is a reefcleaners group buy going on that would be a way to add the clean up crew (cuc) cheap. Add some snails and hermits but I would wait on the shrimp. As for the salinity issue add Ro/Di water daily to account for the evaporation. What you can keep in the way of corals will be determined by the lighting on your tank. Ask as many questions as you can we will be happy to try to help.
The number I shoot for is 30-35x turn over per hour. So a 75g would need 2200-2600gph. The mag 5 seems small for the return. I would think you would want 5-700gph.
Fish on the reef feed all day long. I try to feed small amouts 2-3 times per day. If you figure the amount that you would feed daily and cut it into 3 parts you will not over feed. If you need to cut back then calculate that amount and divide it by the times you would feed. If you miss a feeding everything will still be OK
I would suggest going with T5s and staying away from PCs. I have owned the second fixture and had one of the ballasts go out. T5s give you many more options with corals. You really should not keep anything that needs intense light under PCs.
Buy the tank used. They are like cars they lose value as soon as they change hands. Spend your money on equipment, skimmer, lights, pumps, etc. There are always tanks for sale. JMO
Are you going to have a skimmer in the sump or just use it for extra volume? Either way I would say at least 2 baffles to control micro bubbles from the return pump.
Yes, that is a big change in SG. I would agree that temperature also had a big part in it. I would leave it alone now. Any damage has been done. That large of a change should have been done over several days.