Originally Answered: How to cycle with fish?
I'd never recommend cycling with fish. Do fishless-cycling if you can, directed in this post:
But i know your situation is different, and you're stuck now. Set up a second tank if you can...
Now, having ammonia in your water is extremely dangerous to your fish.
- Do a 50% water change as soon as you can. Use conditioner. Do it slowly and make sure the temperatures of the 2 match. Because your tank is small, and that your fish are already very stressed.
- Then rush to get an ammonia removal filter media or product at a LFS. This will temporarily solve the problem.
- Buy beneficial bacteria product like Cycle or Stability. I can argue about that later, but it works well, but it takes days to have full effect, and make sure you shake it well, and overdose.
Your cycle shouldn't have anything to do with your 0 nitrate. It could be your plant living off the nitrate.
If I were you, i'd use a second tank to do a fishless cycling, then use that filter with the fish once that's done. Using that link above, it'd take you about 10-14 days to cycle a tank fishlessly if you do it right. While waiting the second tank to cycle, you would do frequent water change daily or bi-daily, and underfeed.
If that's not an option, you will have to let the cycle slowly re-adjust.
Every day, do a 25% partial water change siphoning all the leftover food. Put a cap or 2 of Beneficial Bacteria product on daily basis. (Don't use ammonia removal product anymore, else it'd never cycle).
At second week, keep doing the same, but every other day.
At third week, partial change water every 3 days, and stop using bacteria products.
By the fourth week, your tank should have been safely re-adjusted. You can do your water change weekly now. Make sure you never over feed.
Now, let's figure out the source of your problem, why did your cycle broke?
1. Could be your cycle wasn't probably done. Say it's not this.
2. You overfed. Make sure you don't feed more than what they can finish in less than a minute. Twice or 3 times a day.
3. You new load of fish can't be supported. Don't add more than 1 fish every 6 weeks. And whenever you do so, make your water change slightly more frequent, and avoid using much more food than you usually do.
4. You messed up, you forgot to use conditioner.
5. Your conditioner didn't take care of chloramine. Chloramine is commonly found in city water, which is a combo of Chlorine with Ammonia, to make the chlorine last longer. But most conditioner should take care of this.
Anyway, good luck man! :)