there are two basic testers people use.
PH pen: They work well but need periodic calibration. Also the tip of the probe needs to stay wet when not in use. Normally when they are not in use the tip misplaced in a solution of potassium chloride. Even when stored correctly the sensor in the tip will eventually fail, respond slowly or will not stay calibrated. So good quality PH pens have replaceable sensors.
Total dissolved solids meter. Thes pass electrical current through the water to determine the conductivity of the water. The more conductive the water the greater the quantity stuff dissolve in it. Some people use them as an easy quick way to mere GH. Hoveve The General hardness (GH) test mainly measures calcium and magnesium. while TDS meters will measure everything in the water. So It is possible two aquarium to have the same GH but have very different TDS readings.
Beyond this there are color I meters. Thebasic aquarium tests use chemicals mixed with a sample of water and a color comparison chart. In color I meters the color comparison chart is replaced with and electric color sensor. The Cheepest ones on the marketer are Hanna instruments Checkers:
Hanna 717 is a phosphate checker that can m measure between 0ppm and 30pp of phosphate with resolution 0.1. Th
Hanna 713 is similar but with a 0 to 2.5ppm (parts per million) range and a resolution of 0.01ppm. I have the 713 but i now think the 717 is a better choice for fresh water aquariums. They sell salt water versions of these with.
They also sell
HI 775 for measuring alkalinity or KH 0 to 500ppm range with resolution of 1ppm
HI 700 Ammonia o to 3ppm range with resolution 0.01ppm.
HI 746 Iron 0 to 999 ppm (parts per billion) with a resolution of 1ppb. Note this one requires the use high purity Distilled water. My RO DI system wasn't good enough.
Each cost $50. More expensive than typical test kits but far more accurate.
Unfortunately they never released a $50 Nitrate checker. I take that back they just released a nitrate checker but only for salt water aquariums. But still no fresh water nitrate checker.
They do sell higher priced units that will do nitrate or you can buy a desk top unit and bench top units that over a dozen different test but these are basically high cost lab units.