Who Gives A Fish Forums

Just set up a new 20 gallon tank and used a sponge from my old filter to seed it. How long before I can add fish safely? Water params are 0 ammonia, 0 nitrite, 5 nitrate, pH 7.2. Sponge was from a healthy established tank.

If the sponge was fully cycled and you didn't let it dry out, you could add a small bio load immediately. Start with a couple of hardy fish like Zebra Danios to be safe. Monitor params daily for spikes.

I'd wait at least 48 hours to be sure. Even with seeded media, new tanks can be sketchy. Test ammonia and nitrite twice a day. If they stay at zero, you're golden.

That's what I did with my 10 gallon. Added a Betta after 3 days and it's been fine. Just don't overfeed and keep up with water changes.

kyro_1 wrote: ↑Wed Jun 25, 2025 6:02 am I'd wait at least 48 hours to be sure. Even with seeded media, new tanks can be sketchy. Test ammonia and nitrite twice a day. If they stay at zero, you're golden.

Yeah, but if the sponge was healthy, it's kinda overkill. I've tossed sponges into new tanks and added fish same day. No issues if you know what you're doing.

The sponge method works, but it's not foolproof. Depends on how much gunk was in the old sponge. If it was cleaned recently, you might not have enough bacteria. Play it safe and wait a week.

Thanks for the advice, everyone. I'll test daily and start with a couple of Danios if params stay stable.

Good call. Danios are tough little guys. Let us know how it goes. What kind of filter are you running, by the way?

Seeded media is the way to go. I've done this with every new tank. Never had a cycle crash. Just don't overload the bio filter too fast.

My LFS told me to always wait a week, even with seeded stuff. Guess they're just covering their bases. Your tank, your rules, though.