The best way to change more is to get a bigger bucket. The dechlorinator works instantly so you can use the water as soon as you've added it.

OK, that's great. I'll grab some more buckets tomorrow. I've still got the danio in the net within the tank as didn't want him getting stuck again and pecked at by the other fish. Do you think it would be better to let him loose again now, or wait for a day or two until the levels come down? He still doesn't look right.

Replaced more water today and retested, it's certainly coming down now. I've released the danio from the net. He's not swimming right but I can clearly see now that his tail fin is either damaged or missing. Certainly not as much as what the others have. Could that be fin rot?

It could be fin rot, or maybe he's had it bitten by the other fish. Ordinarily, I would suggest medication for this, but it's not practical whilst you have ammonia/nitrite present as you will be needing to keep up the water changes to deal with that. Once those are both consistently at 0 you can consider treating with Esha 2000.

OK, just received the ammonia kit, tested and that looks to be at zero. Tested Nitrites again and that's between 0 and 0.25, so it's still coming down. Overall, I'm happy with handling that in the tank now and feel confident that I know more about the cycle process and how to monitor it.

So my last concern is this little danio and what's bets for him. He's come out from the back, but he's not swimming, more floating. Looks like his left side fin and tail fin are pretty much gone.

Not sure if it's doing him more harm or not to be in the tank loose with the others, so if he's going to be treated in a day or two, would it be best to keep him netted within the tank for now so he doesn't injury himself, or get bitten by the others? Attached a photos where you can see his tail fin is gone :(

That's very extreme fin rot and may explain why he can't balance. Unfortunately been with medication he may not grow that back as it's been eaten away right up to his body. Keeping him in the net is stressful for him, so if the other fish are not bullying him I would leave him in the tank as he may start to adapt to the lack of tail fin and swim more effectively.

On a plus side, the levels in the tank, both nitrite and ammonia are at zero. Just tested nitrite again and that's down too. Obviously will continue to monitor.

The others are bullying him. Watched for a while this afternoon and they keep bothering/pecking him. It does look a lot worse than yesterday. I did put him back in the net as it looked more stressful being pecked at. Not sure what's best now :(

Under the circumstances you describe (ie him being pecked at / bullied by others), and especially if he's not moving that much, being in the net sounds the better option.

Once nitrite and ammonia have been at 0 for a few days or longer, ideally he'll perk up and improve. However, I wouldn't get your hopes up too much.

I bought this for ailing fish when a hospital/isolation tank on their own isn't necessarily the best option but they need protected from tankmates https://www.sera.de/us/product/freshwa ... /sera-breeding-container/ and feed them https://www.hikari.info/tropical/t_13.html which disperses in the water column and enables them to continue ingesting nutrients.