Your ppm's increase due to a build up of nutrients in the growing medium. Your plants NEVER use 100% of the nutes in the soil.. so some of it will dry up and stick to the soil and remain there untill the next time you feed.. then the process repeats and over time the salts will build up to toxic levels. The reason they build up is BECAUSE you are not feeding straight water once in a while. Pretty much every plant food I have ever seen recommends feeding straight water ever 3rd feed if you are using a soil type medium... this is done to keep the salts from building up in the soil... Either way.. give them a good flushing.. like 50-60% runoff with straight ph'd water.. and the next time you feed.. you should notice a big improvement in your ppm's.. cheers!