In a very real sense, fishkeepers don't keep fish; we keep water and bacteria;
they keep the fish!
We should have mentioned this earlier, really, but it essential that you don't overclean your filter media (that's all the stuff inside your filter; ceramic rings, porous rocks or sponges usually) and certainly only ever clean it in old tank water
never under the tap (I hope you haven't done this!).
That's because it's inside the filter that nearly all the good bacteria live. They eat the poisonous, and invisible, substances (ammonia and nitrite) that fish produce, mostly in their poo and wee, but also as a by-product of respiration.
If you want to use the new filter, you can, but you must put all the media from your old filter in it, and then fill the space with new media.
There are loads of good online plant sellers; aquaessentials, Tropica and plantsalive are all very good