Well first off we need to explain a few things first. Diatoms are more a bacteria than an algae. Their cells are actually made of silica. They will normally come in at the beginning of a tank (within months of setting it up) and usually last up to around 3-4 months, depending on conditions.
http://en.wikipedia.org/wiki/Diatom
Coraline (a form of red sea algae) is something that we strive for. It's a good indication of a healthy (at least healthy enough to grow coraline) tank. There are quite a few different types of red algae so it might be better to post a photo and let us try and figure out what type it is.
a little about red sea algae
http://en.wikipedia.org/wiki/Red_alga
Now lets address your tank mates. The sandsifting starfish isn't an adequate clean up crew. These guys eat the good amphipods & copeopods that help break down various detritus and food in the tank. They will clean out a sand bed then starve to death due to lack of food left. Personally I wouldn't put them in anything smaller than a 100g tank due to the devastation that they can cause in the sand bed. Once they do die, they will decompose in your sand bed and possibly cause an ammonia spike.
Your tank will do much better with some snails, crabs and a shrimp. I would strive for 1 snail per gallon rule. I like a good mix of Astraea, Cerith and Nassarius snails (2-3 Astraea, 2-3 Cerith, 9-11 Nassarius) The Astraea snails will clean mostly the glass and the live rocks. The Ceriths will bury themselves in the sand bed and stir the sand and they also help clean the glass. The Nassarius snails like to roam around a lot and eat mostly left over food and detritus. They do go under the sand bed but not for as of period of time as the Ceriths. The Ceriths and Nassarius snails will help keep the upper layers of your sand bed stirred up, which is a good thing to have.
If you like hermit crabs then try and get the Blue-Legged Hermit Crabs. These guys are the more tame and are less likely to interfere with other tank mates. Be sure to get 3-4 extra shells per crab so that they are grow into them as they get larger. Depending on the temperament of the crab, they may go after snails for their shells if there aren't adequate extras laying around.
The flow in your tank is probably adequate. If I recall correctly, the AC 20 puts out like 125gph with the 2 Rio pumps will give you a total around 260gph. This should be fairly adequate for a 15g tank with 25lbs of live rock. Speaking of live rock, I have no idea how you squeezed 25lbs into a 15g tank. I have roughly that in my 29g and it feels pretty tight in there. Be sure to position the power heads and filter outlets in your tank in a manner that provides turbulence and reaches all areas of the tank. I know this isn't easy and may take some time to figure out but you don't want any dead spots because that's where left over food and detritus will settle. If this happens then it might not decompose in the best manner and cause ammonia and nitrite levels to rise.
Feeding will be our next topic. (I know this is becoming a novel) I would cut back on the brine shrimp to once or twice a week. Brine shrimp doesn't contain much nutritional value for a fish. I would get some small pellets (if your clowns are small try and get pellets as small as you can) and check their nutritional information. Compare the labels and I try to get food with as little moisture as possible. Protein and vitamins (A, D & E) are what I look for most. To give you an idea, I'm using Spectrum Grow by New Life. I feed my 2 clowns a pinch per day. You can keep spot feeding the anemone daily if you want, but I would rotate different types of food so they get a variety of nutrition.
The anemone might be a touchy topic for some. Usually you will want to wait to get one until the tank has matured and settled down (6-12 months). Since you have one, just watch your intakes for your power heads and filters. They can get sucked into them and wipe out a nano tank pretty quickly.
Lastly the filtration. You may want to look into a hang-on the back filter. I have a Magnum HOT filter and have ran it on a 20g and it worked great. It uses the same principal as the AC-20 filter but with better results. I'm not saying that you need to switch but it wouldn't hurt to put it on a wish list for later. Just be sure that you are changing your medium in it every 1-2 weeks for best results. You have a choice to run carbon or not but personally I believe that it will only help clear up your water quicker if you do.
I know that there is a lot here to digest. Most of what I wrote is from personal experience from what has worked on my nano tanks. Results may vary so don't take it for the only way to run a tank. Patience is the number 1 ingredient. Nothing good happens fast in your tank fast. If you need to raise a parameter (ie pH, salinity, calcium...etc) do it slowly. Marine fish are more sensitive to conditions than fresh water.
I was going to list a few key things to keep in mind but this article does a good job at it.
http://www.advancedaquarist.com/2005/12/tips
Hope that helps a bit. Oh and before I forget, what kind of lighting are you using?
