I say go for it. Obviously, you care for her a lot. They don't have fish tanks in every nursing home for no reason. This is a great gift for someone trying to enjoy the latter years of a wonderful life, if you ask me. You might have to make frequent visits if you don't already (weekly) to help her with the tank though. I would recommend a 24 gallon nano. When you get into larger tanks you start talking about minimum 5 gallon water changes. Five gallons is a heavy load for an elderly person. For a 24 gallon nano water changes would be reasonable, and she could keep a reasonable amount of fish and corals. I would also focus on getting fish with personality rather than appearance. Some very amusing fish to watch include any hawkfish, tailspot blenny, sixline wrasse, clingfish, and of course clownfish. I use Instant Ocean salt. Though, it is probably more important to use what is readily available to you than what is "best". If you have an LFS (local fish store) that you or her will be frequenting then I would use whatever brand they sell. Most LFS will only carry one brand. Best of luck!