H2O Insider

Water Filtration for Aquariums & Pets

Water filtration for aquariums and pets: chloramine is toxic to fish. RO/DI for reef tanks, carbon block for freshwater aquariums. Product recommendations by tank type.

Aquarium Water: The Contaminants That Kill Fish and Harm Corals

Fish and invertebrates are far more sensitive to dissolved contaminants than humans — they spend their entire lives in the water rather than drinking a fraction of it. A chloramine level that causes no perceptible effect in an adult human can stress fish and damage gill tissue. Phosphate levels below the detection threshold of most home test kits are sufficient to trigger algae blooms that crash a reef tank. The tolerance margins are narrow, and the consequences of ignoring water quality are visible within hours to days.

Most tap water problems for aquariums fall into three categories: disinfectants (chlorine and chloramines), dissolved minerals that disrupt species-specific chemistry (hardness, pH, silicates), and trace contaminants that affect biological filtration (heavy metals, phosphates). The appropriate treatment differs significantly between a freshwater community tank, a planted tank, and a saltwater reef.

Water Treatment by Tank Type

Freshwater Tropical Fish

pH 6.8–7.6 | GH 4–12 dGH | Chlorine/chloramine: 0
Treatment approach: Dechlorinated tap water. For chlorine only: 24-hour aeration or Seachem Prime ($10). For chloramine (confirm with your utility): Seachem Prime or catalytic carbon block.
Products: Seachem Prime ($10) — daily use. Aquatop CF400-UV ($149) — canister with UV for bacterial control.

Most US city tap water works well for tropical fish once dechlorinated. Test GH and KH before adding fish — Amazon species prefer soft water (GH <4 dGH); African cichlids prefer hard water (GH 10-20 dGH).

Planted Freshwater Tank

TDS 150–300 | Moderate minerals for plant nutrition | CO2 supplementation
Treatment approach: Standard dechlorinated tap water is often ideal — the minerals support plant growth. Avoid RO-only unless remineralizing with Seachem Equilibrium or similar. Reduce phosphate if algae is a problem: phosphate remover media or water changes.
Products: Seachem Prime ($10) dechlorination. Seachem Equilibrium ($12) if using RO water for remineralization.

High nitrates in tap water (>20 mg/L) fuel algae in heavily planted tanks. If your tap is high in nitrates, consider cutting 50% with RO water.

Saltwater Fish-Only (FOWLR)

Sg 1.024–1.026 | pH 8.1–8.3 | Ammonia/nitrite: 0
Treatment approach: Dechlorinated tap water is acceptable for FOWLR tanks. Chloramine neutralization is critical — chloramine in saltwater damages biological filtration. Seachem Prime or RO water strongly preferred.
Products: APEC RODI-90-PORT ($230) for serious setups. Seachem Prime ($10) for smaller systems.

The shift from FOWLR to reef typically requires upgrading to RODI water — consider the investment when setting up rather than retrofitting.

Reef / Coral Tank

TDS: 0 (RODI source) | Phosphate <0.03 ppm | Silicate <0.5 ppm
Treatment approach: RODI (reverse osmosis + deionization) is mandatory. Tap water phosphates, silicates, chloramines, and dissolved organics disrupt coral chemistry and feed nuisance algae. Replace DI resin when TDS rises above 2 ppm.
Products: APEC RODI-90-PORT ($230). Spectrapure MaxCap 90 ($250). BRS 6-Stage RODI ($300) — preferred by reef hobbyists for highest capacity and reliability.

RODI water must be remineralized for saltwater mixing — it has no buffering capacity on its own. Use high-quality reef salt mix (Red Sea Coral Pro, Brightwell Aquatics) to achieve correct salinity and mineral ratios.

Pet Drinking Water: Dogs and Cats

Chloramines (cats)

Cats have limited liver glucuronidation capacity and are more sensitive than dogs to chloramine exposure. A carbon block filter rated for chloramine (catalytic carbon, NSF 42) reduces daily chloramine exposure. Relevant for cats drinking 200-300 mL/day.

Recommendation: Aquasana countertop filter ($80, NSF 42 catalytic carbon) or pitcher with catalytic carbon block.

Lead (older homes)

Pets in pre-1986 homes face the same lead risk as humans — they drink from the same supply lines. A first-draw lead test ($179, Tap Score) confirms whether lead is present. An NSF 53-certified pitcher filter (Brita Elite $42, Epic Nano $45) addresses lead in pet drinking water.

Recommendation: Same filter you use for drinking water. Use filtered water for pets if household has confirmed lead concerns.

Plastic bottle water (daily use)

Using single-use plastic bottles for daily pet water is unnecessary and adds cost. Filtered tap water is superior — it's more consistently tested and eliminates leachates from plastic bottles stored in warm environments.

Recommendation: Run pet drinking water through your household carbon filter or pitcher. Use stainless or ceramic pet bowls — avoid plastic bowls that harbor bacteria.

For Reef Tanks: Test Your RODI Output Every Water Change

A DI resin stage that reads 0 TDS at purchase can fail gradually as the resin exhausts — rising to 5, then 15, then 30 TDS over months. Many reef hobbyists don't notice until coral begins to decline. Keep a $15 inline TDS meter on your RODI output and check it before every saltwater mix. Replace mixed-bed DI resin when TDS exceeds 2 ppm — don't wait for visible tank problems. Resin costs $20-40 per pound versus the cost of coral losses.

Frequently Asked Questions