Help with green algae bloom! 🙏🏻
Hello all! As the title suggests, I’m trying to figure out the cause and solution to a recent green string algae explosion in my pond. The pond was build October 2025, roughly 1300 gallons with a 30-32” deep end. All of the bottom is river rock and a few slate rocks I’ve used to make multiple cave systems. The pond has had multiple water changes throughout its time being roughly 300 gallons per change. I’ve only ever used API dechlorinator/ammonia remover at water changes. Only at one occurrence I used the API slime coat water conditioner during a WC. There have never been any other chemicals added. Not a chemical but I did add the Aquascape beneficial bacteria when starting up once a week and then leaving the UV off for 48 hours to let it all get settled and attached. This recent algae bloom started about a week or two ago and it’s the first time algae has been present in the pond. Never had any after creating the pond because we were going into fall with lower temps and changing sun patterns. I have listed every bit of information I could think of below along with pictures to help anyone deduce a potential problem causing this and what could be done to eradicate the algae. I’m not expecting full eradication, just don’t want it taking over like it is. I would appreciate any advice and help to get this resolved. I would like to note that I prefer to not use any chemicals because I’m not wanting to fix one problem and create another. All of the frog bit was just added 4 days ago to help shade and pull nitrates like the hogs they are! The lily pads are really getting going and starting to give good surface coverage for shade. I’m just wondering if all my specs and parameters align with proper levels that it’s just it got warm out and a lot more sun before I could get enough surface coverage to stop the sun enough from feeding the algae. I hope I added enough details below to aid in a solution and thank you all in advance!
Sunlight direct: ~4 hours in spring/summer
Gallons: 1300
Surface area: ~80sqft per my rough measurements
Water temp: 82.8 (at time of typing this 7pm)
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 0ppm (very close to 5ppm, was for sure at 5ppm 4 days ago when I added all the frogbit)
Phosphate: 0.25-0.50 (very close in color on the chart where I couldn’t say for certain, just not quite light enough to call it 0, though in more direct sunlight it could be)
PH: 9.0 (could be higher that’s just the max on the test kit chart, it has never been below this since the pond was built and filled up, even after regular water changes of ~300 gallons)
Livestock:
(5) comets, medium sized between 5-6” (~25) rosy red minnows. At least half of them are 1” or smaller as one had babies not too long ago
(10) Japanese trapdoor snails
(~?) tadpoles from two toads that laid eggs about 2 weeks ago in here
Filtration and water movement:
(2) air stones, one of them being a 4” hygger micro bubble disk and one standard round stone slightly bigger than a golfball that produces medium sized bubbles. 1 is in the shallows and one in the deepest part that’s ~30-32” deep.
Standard pump for canister filter and waterfall rated 1000-1500 gallons with a 32 gallons per minute (1920gph) flow rate.
(1) pump/fountain rated at 3.5 gallons per minute (211gph)
Filter is 2642 gph, UV light, bio balls, with 6 layers of filter media padding