Well the Jeyes is what I use also, something seems really messed up in your water, foam is a really bad sign.
What I have noticed is that if the Ammonia level gets too high the test kit does not read properly - I tried to measure the ammonia in the Jeyes Kleen off directly so I could work out the doseage from that but I didn't get any reading at all.
Only when I put it in water did I get a reading.
I hate to say it but I think you may be better off emptying the tank, cleaning down, gettng the filter into a known state with the sponge and media etc and starting again, unpleasant as it is to say this I think it better for the fish and you in the long run.
It is kind of like a PC problem, do you spend three days diagnosing a fault with no guarantee of success or 2 hours reloading it with a guaranteed result ....
As a side issue in which tank do you plan to add the new fish? the existing one or the new one?
Here's an idea - how about running the two filters in parallel on the existing tank for a week or two, then transfer the new filter and say 50% of the water from the existing tank to the new one (when you do a water change?).
Remember also the time of day that you measure matters - ammonia will be highest in late afternoon or early morning - the light has an impact due to the behaviour of green things (photosynthesis).
what is your pH ?? - Ammonia has a more toxic form at high pH and a less toxic form at low pH, un-ionized ammonia (NH3) and ionized ammonia (NH4+), respectively. In addition, ammonia toxicity increases as temperature rises.
We may be concerned over nothing perhaps, depending on the size of your tank you may not have an issue here at all - it will for example take a long time for 6 fish to upset the stats in a 100 gallon tank.
But I wouldn't use water that foamed as you described ....
