Your water is to high in pH to keep goldfish in.. they really dont' like a pH over 8.0.. BUT dont' worry just yet. Mine in my pond are in a pH of 8.0-8.2 and KH 214.8ppm and GH 250.6ppm,, and they are fine..
This 10 gal tank is going to need to be BIGGER in just a months time.. Goldfish grow FAST.. Be looking now for a 55 gal and DON"T get any more goldfish.. YOUR local newspaper is the best place to find a great deal on a complete tank for less..
PLECO is not needed here in such a small tank, but for now it is ok.. they are messy and get really big!!
NOW, YOUR water changes, HOW DO YOU DO THEM AND HOW MUCH DO YOU TAKE OUT??
YOU seem to be adding in everything in the tank at once. this can burn the fish's gills and that is not good..
IT IS BETTER TO PRE-MIX EVERYTHING BEFORE YOU ADD IT TO THE TANK..
The standard water changes here is like 2-3 gal every week.. NOW trying to get the measures right here on the dechlo/others is a problem as they are meant for a 10 gal dose.. I use a 5 gal bucket then I know the measure to be 1/2 tesp. , but I have more tanks for the rest of the water.. IN 2 gal but under 3 gal, you can safely do 1/4 tesp and be GOOD on your doses.. With your goldfish, they are extremly messy and produce twice the ammonia than other fish.. SO you really need to do every 5 day water changes of 2gal..
NO don't remove them.. Best to remove all the deco out, VAC the gravel.. YES you need to VAC the gravel into a bucket so you know how much you are tanking OUT and dont' go OVER 1/2 of the tank water.. (you can always add some back in once the debri settles)
SALT.. I am confussed here on HOW MUCH you are adding back into the tank.. ARE YOU TOTALLY EMPTYING THIS TANK?? NOT good if you are.. YOU ONLY REPLACE BACK IN, the amount of SALT on the water you are replacing.. SO if you are taking out, 3 gal of water, you only dose for 3gal of water. NOT THE WHOLE AMOUNT OF 10 GAL..
SO TELL Me please how MUCH water you do remove ON EACH WATER CHANGE YOU DO....

EVEN a small 10 gal tank, it stil needs to cycle.. From your readings, NITRITE, it is still cycling and IF you are emptying this tank completely, then the nitrite just comes back.. IF you are not VACUUMING the gravel, this also wil keep the nitrites high.. YOUR NITRATE is high here too. so that tells me the gravel needs to be cleaned.. Something is not right here as you should NOT have a NITRITE READING and a high NITRATE TOO...
WATER: I have a RO system and use it on my American cichlds. I dont'have too, as they seem to do good in the water I have except the high pH.. NOW I do add in some REGULAR tap water to stabilize the hardness so my pH stays stable.. MY pH from the RO is : 7.0-7.2 KH 0-1=17.9 ppm GH 2=35.8ppm MY tap: pH 8.4 KH: 12+=214.8+ GH: 14+ 250.6ppm+
I do however use a 30 gal trash can to mix my water in and a pump to insure mixing.. I mix in 3 to 5 gal of tap water, this helps me keep my RO tanks stable..
IF you want to lower the pH /hardness content, then use more RO and less TAP.. BUT make sure you TEST it first before you add it in the tank. A SUDDEN pH drop or change can hurt the fish.. So beware of that and only do 1 gal every 3 days to ajust them over. THIS WIL NOT SHOW ON THE FIRST 2 TO 3 1 GAL CHANGES BUT REGISTER LATER AT A LOWER pH and hardness. So be aware of that.. YOUR hardness in the tank will over ride the LESS hardness you are adding in until it ajusts more to the LESS HARDNESS. THIS will make the pH come down too but slower/natural way and not shock the fish..
I am unsure here on using the Easy balance BUT it seems it wil work better once you get the hardness down..
YOU really need a 3 gal bucket to do your water mixing/testing in. Make sure on your RO amount to the TAP water you need so when this gets changes over, you know what to do.. TEST AND RETEST the water.. ONCE you get it figured out, you are home free.. EASY then.. Goldfish can do good in a 7.6 to 8.0 so you really dont' need to ajust it much IF AT ALL... THEY do ajust fine to the water they are in and yours have been fine so far..
The problem here is the HIGH NITRITES.. So monitor them and keep them down with water changes of 1 gal every 2 days.. this wil also help the high NITRATES.. KEEP the filter cleaned WEEKLY... NOT just change the filter media pad, but the WHOLE BOX FILTER!! Do so in the sink with a small scrub brush.. Once you add it back to the tank and fill in with tank water, this nutrilizes any chlorine in the filter. This could be keeping the NITRITES HIGH with a dirty filter!! Why clean the tank and leave a dirty filter??