Oh no.....Im walking the dog right now i can't get into this at the moment....im sure someone will be with you shortly.
Edit:
Ok...Im back.
When you say rinsing the filter cartridges you are doing it in tank water right?
I had to read up on Amguard because ive never used it but im a bit confused as to why you need Amguard instead of Prime. It looks like the both detoxify ammonia for 48 hours so if anyone knows and wants to jump in and clarify I would appreciate it.
The best advice I can give is ALWAYS let your
water tests dictate how much water you change. If anyone ever tells you to just change 20% or 30% without any basis behind that decision be weary. How do you know that 30% is enough?
Any time you are dealing with ammonia and
nitrite it is perfectly fine to give your fish as much fresh water as you can....if that means 75% then so be it. Especially if it's rising back up overnight.
If you test your water before you change it, you know what you are starting with.
Then test again after you changed it to see if you changed enough water.
You need to keep ammonia and nitrite diluted as much as possible while you have fish in the tank down around .25ppm is best if not less.
The fact is that as long as those toxins are being produced in the tank, the bacteria will grow accordingly. So if you keep the ammonia and nitrite diluted down, the bacteria is still going to grow and you will start to notice that you don't have to change the water as often because its taking longer for the toxins to rise to dangerous levels.
Soon you won't see the levels rising at all and they will stay at zero because the bacteria has grown sufficiently enough to handle the bioload of your tank.