You need to always insure that what you're using is fish friendly.
For many years we have been recommending ALGAEFIX for algae control. Use only as needed and avoid using this chemical on hot days.
Algae needs two things to grow; a food source (sludge and organic debris in the pond and waterfall) and sunlight. If your pond receives a great deal of direct sunlight we recommend BLACK POND DYE. This will greatly reduce the amount of sun penetration in the pond. The darker you go the more effective it is.
We also recommend MICROBE-LIFT PL for
the overall benefits to the ponds eco-system. Contains live microorganisms that help to eat excess waste in the pond. Effective only when the water temperature is above 45 degrees.
Use these products in accordance with the instructions on the packaging.
All these products are available on Amazon.
Our favorite way to treat these ponds where you are not focused on an eco-system is with a product called FOUNTEC. Fountec is moderately effective in algae control (we generally recommend using at least 3X the recommended dosage), it will also keep mosquitos from breeding in your pond. Safe for your dog and cat as well as the birds. If Fountec alone is not meeting your expectations, consider using it in conjunction with the BLACK POND DYE mentioned above.
Both of these products are available on Amazon.
While none of these products is a substitute for the pond cleaning services that we provide they will help you to maintain a healthy pond in between cleanings.
FIGURING OUT YOUR GALLONAGE
If you are looking to find how many gallons the pond holds then multiply the average length by average width by average depth. EXAMPLE: (15 x 10 x 2) to get 300 cubic feet. A cubic foot can hold 7.50 gallons of water so to find out how much 300 cubic feet can hold just multiply the two (300 x 7.50) to get 2,250 gallons.