I'm not 100% certain on this one. I'm going to try to give you some ideas.
First off look for ants - ants farm aphids and mealy bugs so control the ants to reduce the spread of these pests and to keep them from coming back after you've sprayed.
Some mealybugs get below the soil level and are tough to control without removing the plant to spray.
The way I've controlled them on orchids outside of AP is with a horticultural spray oil combined with water and detergent this combination coats them and smothers them. This sort of thing works with most soft bodied insects but the spray can also coat your lungs (use a mask, you won't notice it immediately). It will also coat the water surface and interfere with oxygen exchange so it's not particularly good for the fish either. This is dependent on contact so if you miss any....
. I think some people might use neem oil to control whiteflies and some other pests. It should work on mealybugs but check to make certain and don't take my word on this. I think CoachChris may have used it and possibly MWDesign (Felix) more recently. Spray only the plants, not the beds or the fish tank - it's slightly toxic to the fish - http://npic.orst.edu/factsheets/neemgen.html
I'd look into beneficial predators if these are available but once an infestation is out of control I don't think these will work as well.