Hi Surfer Joe; Don't let ppm, ec, and PH confuse you. They are not as difficult and confusing as they seem to be. It is just a matter of getting familiar with them and learning what to expect from them.
As THG said, ec and ppm are for checking the level of nutrients that are dissolved in the water. They both are the same thing but check the nutrients in a little different way. Some people use the ec meter as they say it is more accurate but many use ppm meters without issue. I use ppm as that is just what I got used to using. Get you a nice Hanna ppm or ec meter, they are relatively cheap and reliable.
You don't adjust TDS (this is the total dissolved solids that give you ec or ppm). ec and ppm are measures of the TDS. If you start with pure H2O such as in distilled water, your ppm or ec meter will read either 0.0 or very close to it because there are no dissolved minerals in the water. When you mix up nutrients in this water you will then be able to get a reading on the scale or the meters according to how much nutrients you put in.
In hydro, you will mix nutrients into the water for the plants (starting when they are several weeks old from seedling, or when they have become well rooted from clone). The bubbling action does 2 things. It aerates the water so that there is a lot of oxygen present in the water (which keeps the roots from drowning being constantly submerged in water), and it causes the water to splash up onto the netpot when the plants are young and the roots haven't begun to reach down into the water. The clay pebbles absorb this water that is splashed up onto it so that the roots of the smaller plant can get to it.
For feeding in hydro, many people really like the results of using General Hydroponics' Flora 3part nutrient. The 3 parts are mixed in ratio to each other into the solution. This ratio changes as the plants grow and change in needs. Using the ppm/ec meter will allow you to monitor and adjust this ratio as needed throughout the life cycle of the MJ plant. For example; I use Advanced Nutrients Jungle Juice 3part, which is very similar to the GH 3part. I generally begin my seedlings using only 1ml of each of the 3 parts per 1 gallon of water. This gives me a ppm reading or about 350ppm (not sure what that is on ec as I haven't used ec before). As the plants grow, I raise the ppm to 450 then 550 then 650 and so on up to my max nute charge during the vegging period (which can be from 650 for light eating sativas to 950 for heavy eating indicas). I typically will raise the ppm by equally raising the milliliters per gallon during the vegging period, and I raise them each week when the plants are growing well until I get to the max feeding for veg.
Then during the flowering period, I will change the ratio to serve the plant during its bud building stage. I no longer use equal portions or each of the 3 nute parts. Now I raise the Micro and Bloom while lowering the Grow each week until there is no longer any Grow being used and the Micro and Bloom are high enough to get the TDS to around 1200ppm for light eaters, to 1400 for heavier eaters.
At all times when feeding in hydro, the PH has to be maintained in a balance where it is best for the plants. This is a range that goes from 5.3 up to 6.2 with the ideal number being 5.8 for plants in hydro. As all of the chemical nutrients are made up of elements that can range in alkalinity and acidity, The ratio of the nute parts and their ratio to the water will all affect the level of the PH at the initial solution setup. Then as the plants drink water and take up nutrients (never in equal amounts) the PH will "drift" or "swing" as the ratios change. Some of the nutrient brands have "buffers" in them to help stabilize the PH. Some nutes like my Jungle Juice don't have buffers so I have to do the adjusting myself.
This may sound complicated but it really isn't. We can walk you through it when you get going, so don't be afraid to ask questions.
