ok, 6 isn't as bad as 0 but still bad, and if 6 is the lowest on your test kit then it could actually be lower than this as the lowest reading on the test kit means '6 or lower'.
now if the pH drops below 6 then the filter bacteria stop doing their job which would explain why you've had an ammonia spike.
pH can drop over time in a poorly maintained tank, or one that does not have good enough filtration for the waste being produced.
starting to get somewhere now. firstly you need to get the pH up, perhaps someone with more experience of mbu's can offer some guidance on how sensitive they are for pH changes and then we can work out how best to correct the pH.
i asked a few posts back but you didn't answer, can you afford a really big filter like the FX5 in the next few days?