New keepers often ask for a number of days to wait, but the honest answer is that the timeline depends entirely on whether the nitrogen cycle has established, not on the date the tank was filled. A tank running for three weeks with seeded filter media might be ready before an unseeded tank running for six.
Test, don't guess. A liquid test kit that reads ammonia, nitrite, nitrate, and pH gives you the actual answer: the tank is ready when it can process a dose of ammonia down to zero within 24 hours, with nitrite also reading zero. Test strips are less reliable for this specific judgment call and worth upgrading away from during this critical window.
Adding fish to an uncycled tank isn't starting the clock early. It's asking the fish to do the cycling for you, at their own expense.
If you genuinely cannot wait for a fishless cycle, a heavily reduced fish-in cycling approach with frequent, large water changes and very light stocking can work, but it carries real risk and requires daily testing and diligence that most new keepers underestimate going in.
The waiting period is a good time to research your intended stock list, dial in the heater and filter, and, if going the planted route, get plants established so their root systems are settled before fish start disturbing the substrate.
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