Cycle crashed after antibiotics? Fix it fast

Treating the whole tank with antibiotics is one of those situations where you solve one problem and accidentally create another — antibiotics don't distinguish between the bacteria making your fish sick and the beneficial bacteria running your nitrogen cycle. So yes, your concern is completely valid. The good news is a crashed or damaged cycle is fixable. Can you tell me: what are your ammonia and nitrite readings right now, and do you have a liquid test kit to hand? Also, how long ago did you dose the antibiotics, and are the fish still in the tank?

Those readings confirm it — your cycle has taken a hit. You've got ammonia and nitrite both climbing, which means the beneficial bacteria colonies that were processing waste have been significantly knocked back. At those levels your fish aren't in immediate danger, but both numbers will rise over the next day or two if you don't act. The most important thing right now is to do a 30% water change today to dilute what's in the water, and stop feeding for 48 hours to reduce the ammonia load. Then you need to re-seed the cycle fast. Do you have access to any established filter media, or are you starting from scratch rebuilding it?

In that case, the product I'd go straight to is Seachem Stability. It's a live bacterial supplement that contains the actual nitrifying bacteria your filter needs — not just a water conditioner, but the real thing. It works quickly and it's one of the most consistently reliable products for exactly this situation. Dose it daily for seven days as the instructions say, and it will re-establish your cycle considerably faster than waiting for it to recover on its own. Keep testing ammonia and nitrite every day — if either spikes above 1 ppm, do another 25% water change immediately to protect the fish while the bacteria re-establish. You can find it on Amazon UK in 250ml or 500ml bottles; for a 200 litre tank I'd go for the 500ml to have enough for the full course plus a buffer.

That's a fair concern. Bacterial infections often have an environmental trigger — poor water quality, overcrowding, stress — so if you can keep ammonia and nitrite at zero once the cycle recovers, you remove the most common reason fish become vulnerable in the first place. Monitor the fish closely over the next two weeks. If symptoms return, rather than treating the whole tank again consider isolating the affected fish in a quarantine tank and treating there — that way you protect your main cycle entirely. But honestly, getting those water parameters stable is the single most important thing for their long-term health right now.

You're doing the right things and you've caught it early — that matters. Water change now, Seachem Stability daily for a week, test every day, and hold off feeding for a couple of days. Your cycle should be back on track within 7 to 10 days. Good luck with the tetras and angels.