Power cut 18 hours: did my filter bacteria survive?

An 18-hour power cut is a genuine stress test for a cycled tank — the bacteria in your filter media can start dying off after around 24 hours without flow and oxygen, so you're right on the edge of serious damage. The honest answer is: some of your colony has probably survived, but it's been weakened. Before you do anything else, you need to test ammonia, nitrite, and nitrate to see what you're actually dealing with. What size is your tank, and did you do anything during the outage — like manually moving water or keeping the filter wet?

Good news is that the fish looking okay is a positive sign — ammonia hasn't spiked to a lethal level yet. But on a 200 litre tank, even a partial bacteria die-off can trigger a mini-cycle over the next few days, which means ammonia and nitrite could climb without warning. Do you have a test kit at home right now, and if so, what are your current ammonia and nitrite readings?

You've read right — strips are notoriously inaccurate for ammonia and nitrite specifically, which are the two things that will kill fish during a mini-cycle. They'll miss a 0.25 ppm ammonia reading that a liquid test catches clearly. Given what you're facing, I'd strongly recommend getting the API Freshwater Master Test Kit. It covers pH, ammonia, nitrite, and nitrate with liquid reagents, gives you accurate readings down to 0.25 ppm, and comes with enough solution for hundreds of tests. For a situation like yours, it's the one tool you genuinely need.

If ammonia reads 0.25 ppm or above, do a 25–30% water change immediately — don't wait. Repeat daily until it reads zero and nitrite also reads zero for two consecutive days. To support bacteria recovery, Seachem Stability is worth having on hand — you dose it daily for a week and it genuinely helps re-establish your colony faster. Avoid overfeeding completely for at least a week, as uneaten food pushes ammonia up fast when your filter is compromised. The main thing is testing every day so you're not guessing.

Most tanks recover fully within one to two weeks as long as you catch problems early with testing and do water changes when needed. The bacteria colony re-establishes itself relatively quickly if the fish load isn't too heavy. The real risk is the first 72 hours — that's when a spike is most likely to hit. Keep testing daily through this week, keep feeding light, and you should be back to a fully stable tank by the end of it. You're asking the right questions at the right time, which puts you well ahead of most people in this situation.