stop using tap water asap, that is probably where most of your problem is coming from. that and the fact that you have 6 fish in a 55 gallon tank...they can really put out a lot of poop. I used to buy purified water from a grocery store before I bought my ro/di unit and when i ended up testing that stuff it had something like 19 ppm, so that water wasn't even all that pure...for the price you are best off getting an ro/di unit, I got a really nice one for $120 online came with a TDS meter and everything.
also, is your tank near a window? natural sunlight will make nuisance algea grow like gang busters.
here is a list of things you should examine that might be causing brown algea:
-unpure water, should be no more that 5 ppm TDS maximum, fish store can test whatever water you are using if they have a TDS meter, and if they don't...stop shopping there
-natural sunlight...too much will contribute
-too much power lighting...you have about 2 watts of t5 per gallon in a fish only tank, you may want to reconsider this unless you are trying to cultivate coralline algea on the rocks b/c nothing you listed there needs this much light
-too much nutrients...how much are you feeding your fish each day? that has to come out somehow and if the water change water is already high in nitrates you will have a problem
-too many fish, not enough skimming...I have 2 fish in my 55, a clown and a royal gamma, and I use a large in sump ASM skimmer and still have too much nutrients...your seaclone probably just can't keep up with the amount of dissolved organics in your water
-pH and alkalinity...too low of either can contribute to nuisance algea
-flow...poor flow is a massive contributer to nuisance algea, you should be looking at something like 10 to 15 times the total tank volume in turnover per hour minimum, if not much more so something like 850 gph of flow at least
-cleanup crew... you need some stuff that eats algea and you don't have nearly enough for a 55...may I suggest about 20 to 30 astrea snails, at least 15 hermit crabs, and possibily an emerald crab...since you don't have a reef your options are wide open.. and your could consider some type of sea urchin as well
final piece of advice: since you have a fish only tank, if the algea gets too bad, scrub it off, here's how I did it when I had a similar problem a few years back....
you don't want the rock out of water for more than a few seconds and it can't go in fresh water...otherwise either way you will lose bacteria...when you do your water change, keep the old water in a bucket...takes some of the rock out and into the bucket, scrub like mad with a new toothbrush until all algea is in the water...use a second bucket of change water and rinse rock in that to minimize spread of algea spores, then put rock back in tank...once you have all the algea off the rock, the additional cleaners and other steps will help keep it from coming back
good luck, nuisance algea is by far on of the most annoying plagues to all new saltwater tanks, and even us experienced guys sometimes, but you can succeed in getting rid of it
