Black Murky Water and Blanket Weed in a 600-Gallon Pond

Black Murky Water and Blanket Weed in a 600-Gallon Pond

A 600-gallon pond has turned black, visibility down to 6 inches. The owner has a blanket weed controller, UV filter, and has used "chrystals" (likely a flocculant). Blanket weed is slimy. Water tests "okay" but details unknown.

Analysis

• Black water: Could be dark algae (e.g., certain cyanobacteria) or tannins from decaying organics. UV only kills free-floating organisms; attached blanket weed is unaffected.
• Blanket weed: Filamentous algae that attaches to surfaces; UV does not control it.

Action Plan

1. Manual removal of blanket weed (rake, net, or by hand).
2. Natural balance: Add more plants (floaters, submerged), ensure good filtration and aeration, add beneficial bacteria, try barley straw extract.
3. Test specifically for: pH, ammonia, nitrite, nitrate, phosphate, and dissolved oxygen.
4. Reduce nutrient input: Cut back feeding, remove decaying leaves.

Algae resource: FishPondInfo Algae Guide.