Experts and BHG readers answer.
aphids on hybiscus
There's nothing worse than a pest on houseplants in winter. For aphids, you can use a simple solution of dish detergent (a gentle dish soap like Ivory liquid). Spray it on the plant, making sure to coat the aphids' bodies. The secret to control is to spray the plants every 7 days for 3 weeks without fail. You can't miss, or those pesky bugs reproduce and get a jump on your control efforts.
I suggest spraying your plant over the bathtub or a large sink to contain the drips. You can also try using insecticidal soap, which is harmless to pets, but dish soap is cheaper. I recommend buying a small handheld pressurized sprayer (one you pump). Be careful when you fill it, though. Use a small trickle of water or the suds will overflow the top of the sprayer.
You can also spray a horticultural oil, such as Neem oil, if the label says it doesn't harm hibiscus. If you have tried soaps to no avail, try the horticultural oil route. Your local garden center should have a variety of oil types available. Tell them you want one for a houseplant hibiscus.
No matter what you spray, protect the plant from direct sun after spraying (while the solution is still wet on the leaves).
When the weather warms up, get your plant outside. Natural insect predators will keep the aphids in check then.