OK, sorry to break it to you, but there are issues with your stock and equipment
Equipment first, since there is a lot to cover here.
Lights sound good, but how old are the lamps? If un-known, you'll need to replace them, as they may have had their spectral output drift, and output reduce below what they should be. Can you take the lamps out and give us a full run-down of what is on the label? Don't touch the lap directly though, for the reasons Buddyboy gives above
Flow. Quite simply you don't have enough. You need to get at least two pumps, preferably propeller type rather than the usual, that push at least 1800lph each.
Filter. I'd get live rock rubble, Phosphate remover and floss in there if you haven't got that already. The floss with need weekly replacement/cleaning or you may get Nitrate build-up's, and the Phosphate remover will need replacing whenever the algae comes back...
Skimmer. Quite simply the one you've got isn't worth the plastic it's made from. Try get a Tunze or Deltec on there. An MCE600 would be good, and can be bought for about £100 second hand
What salt are you using? What test kits are you using? Can you get an Alkalinity reading? The coral gravel will only buffer the pH to 8.0, no higher
The hydrometers are very prone to drifting and are highly inaccurate. Get a decent refractometer, they cost about £20 new delivered on-line and can save a lot of money's worth of livestock by actually being accurate...
stocking next;
The Humbugs will knack the clowns, other damsels and usually any other tank mates when they become mature, regardless of how big they get. You need to choose between them and get rid of the ones you aren't going to keep, preferably before you scape the tank, as you'll have to take all the rock out to stand any chance of catching them
The main issue with buying a marine tank second hand is that you don't know it's history. It looks like yours was in a very poor state of repair when you collected it, as it looks a mess by way of the algae, equipment specs and water stats. You have a steep learning curve ahead and many issues there to solve, but I'd start with stocking, then the equipment. Water and algae issues can be fixed once you have the right gear, and further issues can be prevented for now by taking out the in-appropriate stock
All the best
Rabbut