The TDS meters should be calibrated using a tiny little screw in the back. There should be a hole where you can insert said tiny screwdriver to change the readings.
New HM Digital models are electronically calibrated via the buttons on the front; much simpler and more precise!
The calibration solution will let me know if my $20 meter is any good! You can't really go cheap on a pH meter as cheap pH meters tend to be inaccurate, some wildly so. I'm hoping that with TDS you can go cheap, we'll see.
My experience with cheap meters is that you might get lucky with a good one that is accurate, or it might be miles off. But even with the 'good' ones I'm not convinced about long term repeatability and consistency of the calibration. But once you have a pot of calibration fluid, you're sorted so long as you regularly check the reading is sensible!
Do you have any recommendation for gently bumping the PH of my aquarium? The SaltyShrimp GH+ doesn't seem to alter it at all, and my aquarium itself is at 6.0 or lower currently. I'd like to get it to 6.2-6.4 so I could monitor it's stability a bit better. I'm using fluval stratum for substrate, and from my understanding using anything with KH will start to exhaust the substrate.
Yes - one recommedation: wait!
Adding a small amount of KH / buffering will temporarily raise the pH, until the soil absorbs it and the pH goes back down again. You can keep going like that until the buffering is exhausted, but then you have no buffering capacity left which kind of ruins the soil. With time the pH will naturally start to rise and should stablise at the levels you want. But adding KH will just cause the pH to fluctuate which is probably worse for the critters than a stable but slightly low pH would be. So my advice is to do nothing (except don't add too much drift wood, leaves, etc that might reduce the pH further).
Update: With the calibration fluid you linked my TDS meter reads 365, so about 23 ppm more than the fluid indicates it should. I have two of the same type, the other reads 354, so closer at 12 ppm more than the fluid indicates. These meters don't seem like they can be recalibrated, however, so I'm going to shop around for one that can be.
I thoroughly recommend this model:
Works great, digital calibration with the buttons on the front (no faffing round with a small screwdriver), reliable, good quality. Cheaper TDS meters do not have temp compensation so, if you're bothered about accuracy (and I'm not sure how much this affects readings across typical aquarium temps), get one with temperature reading and compensation.