Cycling Started Again After Filter Stopped Working Overnight!

One big one gets rid of more than lots of little ones. When you do a series of small changes, you remove some of the new water you added last time, so 3 x 10 litre changes removes less old water (and ammonia) that 1 x 30 litre water change.
80% water changes are fine provided the new water is dechlorinated and warmed to the temp of the tank water. Water changes are stressful for fish but less stressful as having ammonia )or nitrite) in the water)

But don't panic just yet.

The toxicity of ammonia varies with pH and temperature. It is more toxic in warmer water and higher pH.
Go to this site. Leave the top box set to "NH (NH3 + NH4)". Set the salinity to zero, enter your ammonia reading, pH and temp and click 'calculate'.
On the right hand side you'll see two figures, NH3 + NH4 concentration and below it, NH3 concentration. It's the bottom figure you want. That NH3 (without NH4) figure should be kept below 0.05 (yes I do mean 0.05) by water changes and under 0.02 is perfectly safe.


I don't usually give this calculation as newcomers get confused enough with the concept of cycling so saying to keep the ammonia reading below 0.25 keeps their fish safe. But you know all about cycling so this extra information should help.

Since you aren't doing a full fish-in cycle, just suffering a mini cycle after the filter problem, your ammonia shouldn't go very high very quickly. I would keep on monitoring and do a water change if the reading reaches 1.0. Then do a 50% change, which will get it back down to 0.5.
Hopefully, it won't get as high as 1.0 so you won't need to do any water changes.



But just to warn you in case you see a nitrite reading - that is more toxic at low pH than high pH. It can be mitigated by the addition of salt, but you have to be careful of doing that if you have salt-sensitive species such as loaches, cories, rams, some tetras etc. If you do get nitrite and you don't have sensitive fish, I can give you the dosage.

I would not add salt with those fish. If you do get a nitrite spike, keep the level below 0.25. The amount of water to change will depend on exactly how high it gets. If it gets a lot higher than 0.25, you need to do enough to get it down to near zero. If it only gets just over 0.25, then a 30 - 40% change will be enough.

You shouldn't get a nitrite spike. You have a lot of ammonia eaters or your ammonia would be a lot higher. You don't have any nitrite at the moment, so you have enough nitrite eaters for the amount of nitrite your ammonia eaters are making. As soon as your ammonia eaters start multiplying they will gradually make more nitrite, but it'll only be a bit extra nitrite so the nitrite eaters should be able to keep up.

There's always something new to learn 

In the last few months, I have learned about the method of fishless cycling which prevents nitrite getting high enough to stall the cycle; discovered that calculator for how much of the ammonia/ammonium in the water is the toxic ammonia form; discovered the dose rate for salt to combat high nitrite.
I still know next to nothing about plants; Rift Lake cichlids; very large fish; the practicalities of external filters.

And all the things that I have no idea that I need to know about. Yet.