ive never bothered changing my ph, as fish tend to adapt to the local ph when in the fish shop. when i said, how much of the filter media do you change i mean, do you replace everything in the filter, if so, the bacteria that change the nitrites (bad stuff from poo/wee/food) into nitrates (safer but needs diluting by regular water changes normally about 10-20% of the water) will go when you change the the filter media. i looked up the fish tank, i personally wouldnt use it for tropical fish, but rather than replace the filter media, when you syphon the water out into a bucket or something, shake it around in the fish water (dont squeeze it or anything just get excess dirt off) you cannot have tropical fish and have a filter that needs completely replacing otherwise the water will be unsafe. the bacteria in the filter makes the water safe and takes a number of weeks to become in good working order. platties, like goldfish are extremely hardy and therefore will tollerate water (not enjoy) without the bacterial filtration. after a number of weeks when your bacterial filtration is working (this will become clear when you have no amonia or nitrites showing in a water test) you can then get more delicate fish.
once this is sorted then worry about ph if you want to, till then youll only really have the platties and guppies type.
your filter media only really traps the dirt on its own it does nothing more, you need bacteria in the filter media that takes the nasty poisons and turn them into something safe. bare in mind also when you do not dilute the nitrates it builds up and would be like swimming in soup, after a while it takes all the oxygen (oxygen prevents it from reverting back to nitrites) when the oxygen can no longer cope with the nitrates, the nitrates will revert back to nitrites and kill everything in the tank pretty much instantly when the reaction starts, and its not pretty.