Fast Aquatics

Short answer

Standard: 10-25% weekly for stocked tanks. Lightly stocked or planted: every 2-3 weeks. Reef tanks: 10% weekly with reef-grade salt. Nano reefs: 15-20% weekly. Skip water changes only in mature tanks with stable parameters and ULNS dosing - and only after 6+ months of demonstrated stability.

In depth

Water changes accomplish three things: nitrate dilution (the easiest fix for high nitrates), trace element replenishment (calcium, magnesium, potassium, iodine, etc.), and dissolved organics removal (the stuff that causes yellow water and skimmer foam).

Frequency by tank type

  • Stocked freshwater community: 20-25% weekly. Test nitrate; if it's above 30 ppm, increase frequency or check overstocking.
  • Heavily stocked freshwater (cichlids, etc.): 30-50% weekly. High bioload tanks build nitrates fast.
  • Lightly stocked planted tank: 25-50% weekly during EI dosing (mandatory to flush excess nutrients), or 10-15% weekly for low-tech.
  • Reef tank (mixed reef): 10% weekly with reef-grade salt mix. Maintains alkalinity, calcium, magnesium, plus trace elements.
  • SPS-dominant reef on ULNS: 5-10% weekly minimum, sometimes monthly with active 2-part dosing and a calcium reactor.
  • Nano reef: 15-20% weekly. Small water volume = parameter swings happen faster.

The "no water changes ever" approach (popular among some reef-keepers running ultra-low-nutrient systems) requires advanced testing, dosing, and stability monitoring. It's not a beginner approach.

More questions

Can I skip water changes in a planted tank?

Only if the tank is heavily planted (plants outpace nitrate production), low-tech (no CO2), and lightly stocked. Most planted tanks need at least 25% weekly during the active growth phase.