Water Movement in a 20G

Hey everyone. let me first declare my status as a noob reefkeeper.

I decided to switch my FOWLR tank over to a reef system a few months
ago. I currently have a 20g tank, aqualight 130W PC lunar light hood,
maxijet 1200 power head, a secondary powerhead, CPR backpak 2r skimmer
(with rio 600 powerhead) and approximately 20 lbs of live fiji rock.
Not using a glass hood, and I do not have a sump yet (yeah i know, hit
me). I have a fiji sand substrate (bought 'dead') which varies in
depth 1/2" to 2" depending on the spot in the tank. The tank is
currently stocked with a yellow-tail damsel, a tomato clown, 1 red
hermit, 11 blue leg mini hermits, 3 turbo snails and an interesting
small snail i got free as hitch hiker.

I have been testing (and logging) my water parameters they have been
consistently good and stable. I mag-float the front and sides almost
every night. ~20% water change approx every 2-3 weeks. I have been
using the nutrisea water product simply becuase of the convenience
factor and the relatively small size of my tank. Tank inhabitants are
doing fine, - look happy and exhibit no signs of stress or disease.
coralline algae growth has been great since I have started dosing
C-balance. I do not overfeed. The skimmer is producing a very small
amount, maybe a teaspoon daily) of dark green skimmate but i believe
that is due to the low bio-load in my tank.

My goal is to eventually add another 1 or 2 fish (take out damsel), a
cleaner shrimp, an BT anemone for the clown and some SPS and soft
corals down the road. for now i am taking it slow and easy.

the only problem thus far since i setup the tank in June is that I
believe what appears to be the beginning of a diatom bloom. Might just
be deitrus / die-off from the live rock though, as it seems to be very
loose and moves easily. I am not sure if this is "normal" or if I
am perhaps not getting enough water flow in the tank. The brown
dusting is over pretty much all of the sand surface but has not yet
formed a solid surface. I have the maxijet located at the upper rear
left corner of the tank, pointing towards the middle and up towards the
surface to breakup the "oily" film and produce a ripple to aid in
oxygen exchange / circulation. The other powerhead (hagen 301 i think)
is placed on the rear right bottom of the tank, aiming down roughly
30degrees from horizonal towards the backside of live rock.

I am concerned that perhaps the larger particles never make it to the
preskimmer box and therefore are never removed from the system.

I have watched the movement of the water, and it seems that the water
flow is very good to my eyes as it tends to rotate counterclockwise
when viewed from the top of the tank. I wanted to ask the collective
brain trust here at RAMF for advice.

1) will a sandsifter such as a goby help keep the sand cleaner? LFS
says so but..
2) will replacing the 301 powerhead with a maxijet 1200 be too much
flow?
3) does a reef tank need regular vacuuming / siphoning to remove
deitrus?
4) my primary lights are on about 7 hours per day (4-11pm). might this
be too long for a relatively young tank?

This is my first post to this newsgroup. I value any and all advice
given. Thanks,

Malto