Hard tap water is a real problem for fish that prefer soft, acidic water (like black neon tetras). Using distilled water to solve it is logical — but distilled water has zero buffering capacity, which creates a different, faster-killing problem.
What went wrong (ranked by severity)
1. Osmotic shock (primary killer) — Distilled water has almost no calcium, magnesium (GH), or bicarbonate (KH). When fish from the store are dumped into water with completely different mineral content, their bodies can’t regulate themselves. This can kill within hours. Fix: always float the bag 15-20 min to equalize temperature, then slowly add small amounts of tank water over 30-45 min before releasing.
2. Zero KH / pH instability (secondary) — Without KH (carbonate hardness), pH swings wildly in response to CO₂, fish respiration, ammonia spikes, or algae cycling. Even a “good” pH reading in zero-KH water is unstable. Fix: add Seachem Equilibrium (restores GH) and Seachem Alkalinity Buffer or baking soda (restores KH) to remineralize distilled water before adding fish.
3. Incomplete nitrogen cycle — A tank cycled on 1.5 gallons of bacteria can’t handle a full stock of fish. Cycling takes 2-6 weeks with a daily ammonia source. Bottled bacteria seeds it but needs time.
4. Algae oxygen drain overnight — Algae produces oxygen in light, consumes it in dark. A lidded tank with a sponge filter and heavy algae can run low on O₂ by morning.
The fix for black neon tetras specifically
- Target blend: 50-70% distilled / 30-50% tap, with distilled portion remineralized
- Target parameters: pH 6.0-7.0, soft water
- Seachem Prime is excellent but is a detoxifier, not a remineralizer — don’t confuse the two
- Cycle fishlessly first (pinch of food daily as ammonia source), test until ammonia + nitrite both hit zero within 24h
- Add fish slowly once cycled; acclimate properly every time