Algae and nuisance blooms are symptoms. Water testing helps separate "too many nutrients" from "nutrients bottomed out" and from "light and flow" problems.
Start with these tests
- Nitrate and phosphate (low-range)
- RO/DI product TDS (should be 0)
- Salinity, alk
- Microscope check for dinos vs cyano
Patterns
| Problem | Typical water clues | First actions |
|---|---|---|
| Hair algae | Phosphate (and often nitrate) elevated or recently released from rock | Manual removal, herbivores, phosphate export, check RO/DI |
| Cyanobacteria | Can appear at moderate nutrients with low flow, or nutrient imbalance | Improve flow, siphon, review N:P balance |
| Dinoflagellates | Often nitrate and/or phosphate near zero in new or ULNS tanks | Raise nutrients to measurable levels, increase biodiversity; identify species |
More detail on pests: pests and diseases guide. Nutrient control: phosphate, balance.
FAQ
Should I black out the tank for algae? Sometimes for cyano or dinos, but fix root causes.
Can chemical treatments fix it? They may help short term but rarely solve imbalances.