That's too warm for goldfish. You'll have much better success with tropical fish with your setup.
PH: leave it alone. Swings in pH do much more harm to fish health than a steady pH, just about any pH.
Get your own test kits for ammonia, nitrite, and nitrate. That's really about all you need under normal circumstances. Never mind pH. Seriously. You can get a master kit that has ammonia, nitrite, and nitrate tests in it along with a couple of others. Do NOT get those crappy little strips that test for nitrite and nitrate but not ammonia. When you get your tests, follow the directions carefully and post up the results. That will help us see what's going on in your tank, plus you'll need to use them whenever anything seems a bit off.
Filter: Do NOT replace your filter pads frequently. Nitrifying bacteria colonize there; they keep the ammonia and nitrites under control so as to avoid toxic conditions in your tank. When water flow seems a bit sluggish, rinse the filter pads out in dechlorinated water and put them back in. You only need to replace them when they are falling apart or so nasty you can't get them rinsed enough to allow water flow.
Charcoal is only good for a couple of weeks, so you should remove or replace the charcoal from last month. I only use it if I've treated the tank with meds and am trying to get all the last bit of the meds out of the water after having done large water changes. I don't run it on a regular basis.
Do you vacuum your gravel? If not, you need to. A gravel vacuum is pretty inexpensive and easy to use. You can do it along with your weekly water change. Waste accumulates in the gravel and rots, fouling the water.Try to get up under the wood and ornaments if you've not vacuumed before.
Are you still running the corner box filter? Is it the right one for your size tank, and is it your only filtration? I've never had good success with those; I much prefer a good hang-on-back filter or a canister filter. That could just be personal preference, though. Do you still have the fluval? If so, what model is it? I'm totally not familiar with any fluval HOB filters. Good water flow is important.
If you read the beginner stickies at the top of the freshwater beginners forum, you may get some clues as to what is going on with your tank. At any rate, you're sure to find it educational; there's a wealth of information there.