I hate mealybugs. They get down in between the leaf sheaves, making them near impossible to eradicate. Most things that will kill insects will also kill fish, so be careful with any sprays. If you can submerge the plant for a day to suffocate them, or remove the plant temporarily to treat with a short-lived product like soaps or azamax, that can help. Wiping down each leaf with a soap or oil product can knock them back, but besides being ridiculously tedious, it won't get the ones at the base of the leaves (or even around the roots!) where they are known to hide. Lots of things eat mealybugs, if you have a healthy predator population. I don't know of any way to deal with them in situ around fish besides a war of attrition.
