has anyone used the Aquacide pellets before? How did it work?

AQUACIDE is a pelletized formulation of 2,4-D (a type of hormone/phenoxy herbicide). This particular type of herbicide is active on only a limited number of aquatic plant species. So, whether it works or not will greatly depend upon the plant specie(s) being targeted.

Before potentially wasting your money, be sure to accurately ID the targeted plant(s) and verify that it/they are susceptable to 2,4-D. Also, understand that Dec/Jan is definitely not the most suitable time of the year for such herbicide treatments. Above all, procede with caution - reading the product's label in depth before treating. If you don't understand it (some of them can be very confusing) ask a qualified source to interpret it for you.

Finally, be aware that there are two variations of AQUA-CIDE, called AQUA-KLEEN and NAVIGATE, which are much more economical alternatives to AQUA-CIDE. Nevertheless, you'll need to provide a lot more details of your situation before anyone can suggest if ANY of these products are appropriate for you.

how does it work for spot treatments.

most the weeds in my pond is coontail and hydrilla

None of the granular 2,4-D formulations will touch hydrilla. However, these products do a fine job on coontail and waterstargrass, provided that the necessary contact-time (length of plant exposure) is achieved.

It is the need for exposure-time that poses a challenge for using many types of herbicides for "spot treatments". Should currents (of any degree) be present, the herbicide's contact-time will be negatively impacted along with the herbicide's anticipated results.

When using most granular herbicides, consider the granule as a substitute for the water in a water/herbicide mixture. In either case, the clay-base granules and the water are simply "carriers" of the active-ingredient. Restated: just because one applies a granular herbicide to a given area doesn't mean that the herbicidal agent will remain in that targeted area (although the granules remains stationary). The active-ingredient is released from the granule virtually upon contact with the pond's water. At that point, the active-ingredient is free to move about at the whim of any existing currents (whether generated by wind, thermal or rain induced forces).

I have sometimes used dyes in an attempt to determine the presence of currents which may go undetected by normal observations. Place a few ounces of lake-dye in the area being considered for such treatments. If the dye remains confined in the area for several hours (up to a couple of days), then a granular 2,4-D spot-treatment may have a shot - provided that all variables remain comparable to the period in which the dye-observations were made. However, if the wind kicks up all of a sudden - prompting currents that didn't exist during the dye observations - all bets are off for the herbicides performance.

One last comment: 2,4-D formulations work best on susceptible plants that are in a very active growth-stage. Their performance is greatly reduced if the product is applied either too early or too late in the season.

The above comments are very "general", and should not be viewed as guidance for your situation - since I know very little about the scenario that you're facing. More info about your pond (location, size, depth, degree of infestation by each specific plant specie, flow/current dynamics, etc. etc....) would be needed before making any actual suggestions.