The doughnut shaped Bt called "MosquitoDunks" is good for 30 days like you indicated. I believe the Bt stays near the surface of the soil so, every 30 days should be enough. However, I apply every 2 weeks just in case. "MosquitoDunks" instructions claim that 1/4 of a dunk will treat 1-5 sq. ft. For instance, my 3 gallon, 10 inch pots have about 78 sq. inches at the surface. So, 1/4 of a dunk should treat nearly 10 pots. Personally, I would use at least 1/3 or even 1/2 of a dunk for 10 pots. I feel safer using the stronger dosage. Using the 10 pot example again, I would dissolve that 1/3 of a dunk in about 2 gallons of water and try to roughly use equal amounts on each pot. You may use that same 1/3 dunk with 3-4 gallons of water as long as you use it on 1-5 sq. ft. and in equal amounts in each pot. I don't leave dissolved Bt unused very long. I like having the Bt in the pot for it's 30-day life span rather than sitting in a container. This stuff is pretty tame as for as toxicity, so don't get all bent about exacting measurments. Never heard of Bt harming plants regardless of dosage.

What an interesting pond you have inherited. Are you going to keep it as a pond or camoflage it as a water garden? I have heard that ranchers put mosquito fish in the cattle water tanks. They don't have filters but they do have a fresh water source. They take them out in the winter I think. I wouldn't want to worry about the fish though. I think dunks are a better way to go. They are natural and I wouldn't get emotionally attached to a bacteria, even a helpful one. I copied this info from the link below. "The second most effective way to eliminate mosquitoes is to treat pools of standing water with larvicides (agents that the kill mosquito larvae). Larvicide agents include vegetable or mineral oil, insect growth regulators such as methoprene, bacteria such as BTI (Bacillus Thuringensis Isrealensis) and BS (Bacillus Sphaericus), and fish that eat larvae. BTI is readily available in a product called Mosquito Dunks. While BTI and BS are not toxic to mammals, they may be harmful to non-target insects and mosquito predators." Here is a link that might be useful: Mosquitos in Pima County

Washing will not prevent an infestation of fruit flies, if you bring them in the house in the form of eggs that have been inserted under the skin of fruit. Washing won't even prevent egg laying from visiting fruitflies. In the home, you can prevent future problems by wrapping the individual fruits in paper towels or napkins for counter storage or putting them directly into the 'fridge. Karen, are you certain that you have fruit flies? If so, you can very successfully trap the adults by putting a bit of apple cider vinegar and some dish soap into a small container with a little water for dilution. The adult flies will flock to that solution and sink to the bottom. This will only work on fruit flies, not on fungus gnats or drain flies. It's a very simple and effective remedy. Bt will not be effective against fruit flies.

I have never used them myself but I know "Bacillus thuringienis israelensis", the active "ingredient" in Mosquito Dunks will kill fungus gnat larva it is comes in contact with them so I see no reason why it should not control a population in your bin. I would use 1/2 a dunk and soak it in the recommended amount of water and use this treated water to soak some bedding in and mix it into the bin. Fungus gnats like wet conditions so after application I would let the bin dry out on the surface to further discourage any remaining adult gnats to lay eggs back into the bin. I had a gnat problem the first summer I had my bin. I think they started in the worm bin and quickly spread to all houseplants and then started swarming in the kitchen. I applied beneficial nematodes to some coco coir and used it to top dress all plants and dumped the rest into my bin. I have not had any fungus gnat problems since. I believe Bentley is experimenting with dunks on his blog now and may have more/better info... Here is a link that might be useful: Can Mosquito Dunks Kill Fungus Gnats and Fruit Flies?