(1) I used to run my power compacts 11 hours per day and I never had any problems. You don't NEED to restrict your light to 5 hours per day - many of us do this to conserve money on our electric bills. Think about where reefs are located - in those places, the corals are going to have light from the sun for somewhere around 12 hours per day. By only leaving our lights on for 5 or 6 hour, we're creating strange subartic reefs, sunlight-wise.
(2) Definitely switch from purple up to B-ionic 2-part to dose for your calcium and bicarbonate. However, since you only have a few softies in your tank, I doubt you need to be dosing daily. Check your Ca and Dkh on a particular day, then check it again the next day. That will tell you how much calcium and bicarbonate your tank is using on a daily basis and adjust your dosing acccordingly.
(3) Read Matt's "Cyanobacteria Primer" which has all the information you might need about cleaning up your red slime problem.
(4) Just because you are not using pellet or flake food does not mean you don't have phosphates. Unless you are feeding all live foods, any kind of food you put into the tank is probably going to add phosphates in some way. The trick is to do this at a low level and then remove them.
(5) I would add GFO and carbon to your system to help with nutrient export. You can do this with phosban reactors (best option) or at the very least, put it in filter socks in your sump (less good option).
(6) I would feed the tangs more than just a little seaweed. They need their veggies in order to have a completely balanced diet. Lack of veggies can lead to nutritional deficiencies. Some people have suggested that this might cause HLLE.
(7) Once a tank is set up, you will never have a sparkling clean sand bed ever again. However, if you want a clean-ish sand bed, you should stock things that will sift the sand for you. Nassarius and cerith snails would be a good start. Once your sandbed is more established, you could add a sand sifting goby - but be aware that they will bury your corals.
