Well firstly, that lionfish is going to be messier than most fish kept in that small a tank. That can add to a higher nitrate level/phosophate level in the tank, and may cause algae to show up in the future along with having poor water quality. Too small a tank IMO for a lionfish.
that said, if tank is that old, bulbs that old, and using ro/di, probably just the move that caused the algae to show up. stirred up too much gunk in the tank. Stick to water changes and normal maintence, and this should clear up in time.
fwiw, alot of test kits out there do not give very accurate readings. Test strips are a joke, and API (brand from petsmart, walmart) will ballpark the numbers close, but still can be inaccurate. Salifert seems to be the "prefered" brand but never used them myself. If validity of the tests are in question, you can run a water sample to most LFS and they'll test the water for you. Or find a fellow reefer close by who would be willing to give you a second opinion on your results.