i have had goldfish for years, and although i wouldnt say im an expert, only reason i have lost fish is because ive added one that ended up being sick and killing the tank
didnt want the thing anyway, i was persuaded.. anyway... as said, salt, doesnt really help goldfish much in general and really more a tonic, if they get sick. be careful with salt too, if you ever add it in the future for medical reasons. i believe your fish be ok, but dont hold me to it, im no expert on them specifically, but some fish, dont handle salt well. a few sucker type fish dont for instance. so just future reference, check before adding.
if your filter uses filter cartridges, ie, with carbon in them, change them monthly at most, as carbon can have reverse effects if left unchanged for too long. any 'bio media', such as bio wheels or bio pads, shouldn't be changed, again as said already. just rinsed to remove loose stuff. you want the bacteria in those.
gold fish pretty clean fish, they good at house work it seems, always cleaning the floor, so pretty easy to keep their tanks clean. sounds like you doing it right. if you have any decorations in there, move them and vacuum the gravel under them every now and then, as it can build up and cause issues.
mine love the bubbles from the air stone. always playing in them. the monument type thing has an airstone in mine, but it produced large bubbles so i ended up getting an air stone bar from walmart, only couple bucks, and putting it under the gravel in the back of the tank, then a small air pump to power it. i also added a regulator to it so i could turn the air down a bit, else it produce a wall of bubbles, but if you get a small one, should be ok. try one of those, you may find they enjoy it, if you dont have one.. oh make sure you get a small air check valve too so water wont back up into the pump and so forth. again they a couple bucks from walmart.
finally, and this is just from my experience, others may disagree, but i have always refrained from using any chemicals in my gold fish tank. i actually do very little to that tank and my readings are always pretty much spot on. all i do is use stress-coat when adding or changing water, to remove chlorine. compared to the tropical and salt tank also running, gold fish one is lovely in terms of maintanence. they just keep swimming...
you will upgrade the tank, if the fish need it or not. its just the common rule of fish keeping.. theres always a bigger tank calling your name! lol