What can I do to treat trapped fluid in my ankle?

Water falls... Since it's only in one ankle, it's either trauma induced or something else is going on further up the leg, causing an impeded venous return. Either way, begin by elevating the foot higher than your heart. To remove all doubt, consult with a primary care physician, either your own or consult one at Healthtap.
DIAGNOSIS. You have to find the reason for the "trapped fluid." Then, it can be treated correctly. Also, you said ankle, not ankles. It's only one ankle? I would start with your primary physician. Sincerely, Dr. Latva.
Edema. Edema in the ankles is common. Best to elevate feet and legs. Use ace bandage or otc compression stockings 15-20 mmhg.