My Harmony catalogue has Mosquito Dunks listed as effective in ponds. Here's what it says about them:

Doughnut-shaped Mosquito Dunks float on water and slowly release bacillus thuringiensis - a natural pesticide lethal to mosquito larvae. Place all-natural dunks anywhere water is left standing for more than a week. Safe for birdbaths, rain barrels, ponds, ditches, tree holes, roof gutters, unused swimming pools, and anywhere else water collects. A single dunk destroys mosquito larvae in up to 100 square feet of water for a month. They are $12 per 6 in my catalogue. ANd, they also have other repellents listed.