How often have you changed the PURA pads? The larger pad sounds like a good idea, maybe a couple of them. Extra pads would be a whole lot cheaper than all that water. It was recomended to me to change it out every 3-4 weeks.
Check into the nitrate sponge as well. It can make a difference over time. Suggest not adding any more to the bioload until you know where your at for sure.
Even with a large system like your you need to give it a little time to adjust to new additions and build up the extra nitrifying bacteria.
Good luck,