So a few thing that I hope a few other members end up reading:
1- Just because a filter says X GPH or X tank size doesn't mean that it will actually have that much flow or support that size of a tank in every situation. Canister filter ratings are usually based on the flow with minimal head pressure (the height difference between your filter and your tank) and with no media installed. once you add media and 4 feet of head pressure (average if your filter is under your tank) the flow rate can decrease drastically. Whether or not your filter is adequate depends on your water chemistry which leads me to....
2- What are your other water parameters like? Knowing what your ammonia, nitrite, and nitrate levels are will help you asses if your filter is up to the task of filtering your tank. What are those levels in your tank currently? You said your ph was very low. Are you using anything to alter your ph? You said your water was soft, are you using RO water? With ph that low, it indicates to me that you might be using RO water, and also high nitrates. I could be wrong, but ph doesn't get that low unless you want it there, or there is something wrong with your water. Which brings me along to...
3- What is your tanks maintenance schedule like? With the large severum you should be doing at least one %50 or more water change a week along with gravel vacuuming.
Once you take all of these things into consideration you can make an educated decision about whether or not your tank can support another inhabitant.
