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So alk and calcium are the foundation for coral growth bu they have no effect on algae growth. Nitrates and phosphates are what cause algae to grow. So get all of them in check and your sps will grow.
I am a huge fan of always using a new sand bed when you move a tank, we have done it several times and have never had a problem doing it. When you disrupt and old sand bed you can be realeasing a lot of nasty stuff, and if you don't properly rinse it (like Mark states above) you can do more...
Tank looks amazing. If you want a cheap "hack" get some of the orange gel filters to take your pictures through. Might make a big difference and it is a really cheap option.
Sump flow really depends on what you have going on in the sump. Is there anything else beside a skimmer in there, any baffles, any other reactors or chaeto or whanot?
I would pull the bigger fish and do what you want with the tank. This hobby is tough enough to have a tank you don't really like. Also those fish will get really big for a 120 and might cause even more problems down the line. Get your teenaged son involved and get him hooked and you will have...
Will always have a spot in my heart for a 20g high, it was my/our first tank. One of the nice things about it as you learned is it is very easy to hit the reset button on. We never drilled ours either and just ran HOB fuges and skimmers. Worked out really well.
Looks really nice
I believe the conclusion to the study was that it was fine to start fish on selcon to get them to eat frozen foods but you don't want to do it long term because of the liver problems. If you are feeding LRS, your fish are getting a good quality food and you should be good to go from there.