That's great to hear about the tang! Just keep an eye on her for any signs of stress. Mine was a pain to catch. She would hide "underneath" the rocks. I pretty much had to take ALL of my rock and coral out of the tank to finally catch her.
With regards to rock/coral placement.....this is the fun part.......just be creative. When I put my tank together, I intended on having crabs and shrimp, so I put some live rock together in the shape of a cave so they can hide with lots of live sand with crushed coral so my pistol shrimp and goby could burrow. With all of the other rocks, I just rearranged everything else until I was satisfied it looked nice. Plus, if you add more live rock and sand, it will be beneficial to your eco system. I would also visit my lfs, and arrange live rocks in different patterns until I found something that would look pretty cool in my tank.
About the green hair algae, once you decrease your nitrates, you will have better control. High nitrates fuel this problem. I highly recommend you start combating this problem by placing a protein skimmer on your tank to remove dissolved organic compounds.
Since your nitrates are not through the roof, I recommend you begin with a 20% (~8 gal) water change, followed by a 10%. I'm confident that if you add a skimmer, along with the water changes, your nitrates will significantly drop. I run my skimmer 24/7. Also, you can place some dragon breath algae in your tank. It looks really cool (especially at night under the blue lights) and it helps lower nitrates. Many fellow hobbyists place some in their refugium. I used to have a bunch in my tank until I got some emerald crabs. They had a feast with this stuff! You can also add some NO3 PO4 biological nitrate and phosphate reducer. This stuff works great! It will quickly lower your nitrates, but you will need to ensure to address the problem. Along with the skimmer and water changes, I feel you should have your nitrates under control in no time. Also watch how much you feed. Although, I don't think this is the problem in your case. Once you lower your nitrates, add some more "cleaning crew" and this will also help.
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Premixed water huh? Is it expensive? I started by mixing my own with RODI, but I quickly transitioned to using seawater from Scripps Aquarium. They offer the filtered saltwater used for their aquariums to the public for free. In any case, the water you are using should be fine.
Sure, you can add another powerhead! Just be careful on the flow rate. Too much flow in your tank can be detrimental. And "yes, yes, yes" on the skimmer!! It's a must. There are many HOB skimmers to choose from. I use the CPR Bak Pak. Many people do not care for this skimmer, but I've never had any problems with it. It's been working like a champ, and each week I get a lot of skimmate. However, it doesn't perform any surface skimming, so you will get a light oily film on top. If I were to purchase another, I would get the Tunze 9004. It can be placed inside your tank, it's super quiet, small and it performs surface skimming. It has received many positive reviews and it will not break the bank.
A sump is a great addition, but I don't feel they are always necessary. I've been doing fine without a sump or a fuge for a year now. To add a sump to your tank you would need an external or internal overflow box. The external boxes are not very popular. Over time they have the potential to leak all over your floor. With an internal overflow box, you would need to drill your tank. Not sure if it can be done with a bowfront (tempered glass?). If it can be drilled, unfortunately you will need to drain your tank completely. I plan to upgrade soon and my next tank will be definitely be a reef ready (drilled) tank. I will add a sump then.
Hope this helps.......and good luck!!! Keep us updated
Oh yeah....what's your favorite fish so far? Mine is the yellow watchman goby. I love to watch my goby and pistol shrimp work together digging holes all over the place......actually, the shrimp does all of the work.
And the one I dislike most........domino damsels. They are some of the meanest fish I've encountered. The one I had would bully every other fish in the tank....no matter how big they were.