When you treat for ich, it should be either heat or medication not both. Medication and heat both reduce the amount of oxygen in the water. Paraguard's instructions are for a range of conditions; they don't say anything specific for ich, so I would continue the treatment for 7 days after the last spot disappears. The danger with ich is stopping treatment/turning the temp back down too soon. The parasite can only be killed in the third stage of its lifecyle, and the spots on the fish are just the first stage.
You probably know all this but just in case -
#1 is the spots on the fish. The parasites are feeding on the fish. They are inside a coating where meds/heat can't harm them. When they've eaten enough, they fall off the fish.
#2 is when the parasite is inside a sort of cyst on the bottom of the tank. Again, the coating protects them. They multiply and multiply inside this cyst till there are hundreds of them.
#3 is when the cyst opens and all those new parasites go swimming off to find a fish to infect. Now they can be killed - and only in this free swimming form.
So there must be medication in the water or the heat still at 30 deg C/86 deg F when the very last cyst opens.
Once treatment has finished, the med will be diluted out with water changes. You can also use brand new carbon in the filter to adsorb it. Throw away the carbon afterwards. Speaking of carbon, you did remove any carbon from the filter before starting treatment?
When adding a new filter, it won't increase the number of bacteria in the tank. The amount of bacteria is dependent on the amount food so same fish, same number of bacteria. Just letting two filters run will take weeks, if not months, to grow bacteria in the new filter. And they'll be spread between the two filters.
What to do depends on your plans for the filters.
If you want to run both together indefinitely, just add the second filter and leave them both running.
If you want to remove the first filter, the simplest is to take the media out of the first filter and put it in the second. The old media can be persuaded to fit. Sponges can be cut up. Ceramic media can be mixed in with the ceramic media that come with the new filter.