Good for the filter cartridges never being changed till they fall apart. If you can squeeze two cartridges in there back to back/ I call that piggybacking them, you can develop twice the BB which will help you keep the tank stable.
Right now or asap I would change 50% of the water, always using the water conditioner of course (every time) Then see if the fish doesn't calm down.
If you can take a before water changing zip bag of water and also a sample of after the water change, to the lfs and have it tested, you might see some unsafe parameters on the before, and also get the numbers from them.
Also with the before and after you should see better numbers on your after of course, we could know if he is agitated due to toxic water conditions.
When you have a test kit, a master test kit, you can check this all yourself.
One issue might be the newt creature is stressing him out. The Newt might eat your adf. You might want to check into that a little further. They pretty much eat what ever fits into their mouths and they can open pretty wide. It is in their nature for forage for food.
Because you changed out the filter pads where almost all the BB live, it will take a number of weeks to get that in line and cycle your tank.
Here is a thread for you to read
Fish-in Cycling: Step over into the dark side - Aquarium Advice
Also there are a lot of articles on the articles section of the AA forum.
It is on the main home page along with Getting started.
If your parameters are bad you will need to do another pwc probably 50%
They can test for Ammonia, Nitrite and Nitrate, pH and alkalinity.
The Platy could be just checking things out but if you don't know your parameters, they could be suffering toxic effects from one or more of the above issues. Clean water can change that problem.
Does he have any white dots on him, stringy whitish poop, fuzzy patches, grey white patches, any thing else which isn't quite right, dark bright red or grey brown gills cloudy eyes???