Hey there Whiskey... I copied this from the internet because I am not that great at explaining and end up leaving out a few steps. Hope this helps!
**Beginning of copied thread
If the ammonia is under 0.25 then you are okay. If it's between 0.25 & 0.5 then carry out a sizable water change, I would say no more than 50%. If your ammonia is very high, 1.0 and above, carry out a very large water change, 75%, 80%. Many people do a water test directly after the water change. Sometimes you can get false readings doing it this way so always carry out your water tests before your water change. It's not worth panicking about, your water change should remove the ammonia so don't worry.
I'm not going to commit myself and say how long it will be before ammonia returns to zero, but at some stage this will happen, in the meantime, you will notice your nitrite getting higher, treat this exactly the same way as the ammonia. Finally, you will start seeing a nitrate reading. Nitrate is a byproduct of ammonia and nitrite and is not toxic like ammonia or nitrite. Once your ammonia and nitrite are zero, you have established a biological filter and your tank is cycled. The whole process could take six or seven weeks so be patient. Try not to panic during the cycling process if one day your ammonia is higher than the previous day, if you just carry on monitoring the ammonia and nitrite and do water changes accordingly, eventually the bacteria in your filtration will buildup to the extent that you will not have any ammonia or nitrite present, but you have to remember it will take several weeks if you are just using a few small community fish, so be patient. Do not clean your filters during the cycling process, unless you have some very fine polishing pads that get clogged up very quickly.
end of copied thread**
Only thing I can add to this posting is, Nitrates are dangerous to fish in high levels but the affect on the fish is not immediate. Nitrates should be kept at 0-20 ppm. Anything over 20 needs attention, which would be... water change, could be overfeeding, substrate may need better cleaning, decorations that have poop and left over food in, around or underneath them, overly dirty canister filter and I would guess a dirty HOB filter (we don't use HOB's - hang on back filters) and I'm probably leaving something out so someone else can elaborate :) Hang in there, we all go through this and it doesn't last that long. Before long you will be kicking back with your Whiskey (LOL) enjoying your fishes :) :)