I have a very small hydro setup: One Waterfarm in a C13 cabinet (got the instructions from Overgrow and it worked out perfectly).

I have never owned a TDS/EC/pH meter. Instead, I bought a cheap little pH testing and nutrient kit from the local garden store. I follow the directions on the nutrient bottles and only test for pH once a month or so, just to make sure the pH from the tap water isn't changed. Once the nutes are added to the water, I'll get a reading of 5.5-6 (it's color coded, so there is no *definite* answer), which is about where I want to be.

I used to use pH Down to drop the pH even lower, to around 4, and it didn't hurt the plants. But, I figured that it sure as hell wasn't helping anything, either. Got 5+ ounces out of one harvest even when I was using water with a pH of 4.

My point is: If you can't/don't want to buy a meter, then don't. You can buy the pH testing strips instead for like $8 for 20 strips. That's more than enough for the testing of a couple harvests.

If you have a large grow, you probably should get one of the meters, though, since there would be more chances for nutrient/pH problems to occur.