Things to add to a 20G and algae/sludge questions

[naturenutty]

Hi all! I'm relatively new here. I signed up a long time back but I don't think I ever posted. I'm really a newbie to SW. I got an established tank last fall (wow... almost a year ago) from a friend. It has been established with pretty much the critters in it for at least 5 years. I have kept FW fish for a number of years, off and on.

I've had a bit of an off and on problem with green algae forming a slime coating on the sand/rocks and glass. I was doing water changes every 10-14 days, and my guess is when it goes 14 it's just a bit too much. I'm now going to make sure H2O is changed every 7-10 days, and I also have recently decreased the temp and photoperiod and I'm hoping that will help solve this. Any other suggestions and ideas are welcome. I won't be going to RO, though, so even though I know that will be something everyone suggests, it isn't happening. hehehe

There also seems to be a continuous greyish white sludgy/silty substance that covers EVERYTHING when I clean the tank. Am I just disturbing the bottom too much? Not enough?

My other problem, if I can call it that, is that I need to add something to it and I'm not sure what. It just needs a bit more... something. hehehe Anyhow, here's the specs on it, and any and all input will be much appreciated.

-20g tall
-live sand bottom
-live rock (really not sure how much, I never understood how it's weighed, but there's a fair bit)
-Aquaclear mini with filter floss
-Rio Nano skimmer that sometimes skims (if ANYONE has a solution for this, please let me know... I've found this to be the most finicky thing I've ever dealt with)
-Coralife fixture with actinic and 10K tubes. (10K tube is due for a change. I've heard actinic can run a long time without change, is this true?) now on for 10 hours/day (down from 12)
-Temp around 24C (down from 25-26)
-submersible fan-type power head that I move around to 'blow' in different ways when I'm inspired
-I use tap water for water changes, treated with Prime and Instant Ocean to 1.025 specific gravity
-I add Kent Marine nano reef A/B about 2-3 times a week

home to (might have names wrong... I apologize):
-a small herd of snails
-an unknown number of tiny brittle stars
-2 small and 1 large blue-leg hermit crabs
-did have 2 peppermint shrimp, but they haven't been seen in a few weeks, so I'm presuming they've expired
-open brain coral
-small finger leather coral
-green star polyps
-a collection of coral fragments that hang out on the bottom of the tank and make me wonder what they are
-assorted other unidentified salty objects (USOs)
-Royal gramma
-yellow-tail damsel

I feed NLS pellets almost daily (with occasional fasting days), a liquid food for the reef critters a couple of times a week, and once in awhile I put in bits of frozen brine shrimp.

I'm not sure if I'm missing anything important there.

Things I've been thinking of are low light easy keeper reef type organisms to add some colour and/or movement and general interest to the tank and something to help sift/clean the sand a bit... maybe a goby? I actually had a person from what I believe is a fairly reputable SW store tell me that I could have a few more fish in here, though I thought it was best to keep the numbers low. He succeeded in getting me to take home a goby, which lived all of 2 days before it died of unknown causes (salinity was fine, ammonia and nitrites 0 and nitrates barely measurable, I did the slow acclimatization, and was feeding it some frozen brine shrimp daily on the advice of the salesperson). I'm a bit nervous about adding another fish now.

ANYHOW, I'm looking forward to learning from you all. I will be reading through as much as I can, but I thought I'd throw out my situation for your perusal and input.