I have no idea how to cure a goldfish tumor, but I can probably help you get your nitrate levels down.
Your 75 gallon tank could conceivably handle 6 small goldfish for a few years until they grow too big for it (comet goldfish can get over a foot long and you'll need to either rehome them or install a small pond), but only if you're using the correct filters and media.
The key to keeping down nitrates is cultivating a large colony of nitrate-eating bacteria. If you're using those little blue filter pads filled with charcoal that you're supposed to change every month, you're doing wrong. You're literally throwing away your bacterial colony every month, and paying stupid amounts of money for the privilege.
To keep your nitrates down, you need four things:
The biggest filter you can afford; Canister filters are best but this will work for hang-on-back filters as well.
A bag of medium-sized plain lava rock from the hardware store (the kind used for grilling); you want pieces the size of walnuts/pecans. If you can't find small ones you can break the big ones up with a hammer but be careful because the pieces will be hella sharp.
Plain poly-fiber media padding.
Seachem Stability Aquarium starter (technically optional but highly recommended).
If everything goes well, your water should now flow through the poly-fiber media pad, through the lava rock, and back into the aquarium smoothly with little to no reduction in flow. If they water's moving too slow, reduce the amount of padding/rock.
Maintenance: Rinse the padding once a week in de-chlorinated water to clean off the gunk. Rinse your lava rock every six months or so in de-chlorinated water to clean off any silt. Do a 75% water change once a week. It's a myth that large water changes are bad. The bigger and more frequent water changes you can do, the better (I recommend using a <em>Python</em> for large water changes).
Finally, I recommend feeding your goldfish only once a day, every other day. This seems cruel but it's not, the goldfish will survive just fine this way, and they will release much less waste. Goldfish are like dogs, they constantly beg for food regardless of whether they're actually hungry or not.
If you do all this, you should see your nitrates go down to acceptable levels.
Edit: I also recommend using a self-starting gravel vacuum during water changes to suck the gunk out of your gravel. Accumulated food/fish waste in the gravel can cause nitrate spikes.