Reservoirs should be emptied and the nutrient solution replaced at least every 5-7 days. People often use their EC/TDS meters to measure nutrient levels and top up their nutrient solution when levels get low. EC/TDS meters only measure overall salt levels, not the levels of specific nutrients. This means nutrients not completely used by your plants will begin to build up to potentially toxic levels. Emptying your reservoir and replacing your nutrient solution with a quality fertilizer is the only way to ensure nutrient levels are consistently in the ideal range for plant growth.
There are many supplements that will increase flower size. Most common are bloom boosters that are concentrated in powder form and are high in phosphorus. Bat guano supplements also increase flower growth and I also really like foliar spraying with folic acid, often labeled gold. This will increase the number of flower sites and act as a chelating agent, helping your plants absorb macronutrients more efficiently.
That scummy, sludgy buildup in your reservoir could be algae, bacteria or fungi. These organisms rob your reservoir of oxygen and nutrients, leaving nothing for your plants. They clog pumps and drippers, causing nightmares in the growroom. Regardless of what it is, it is usually caused by light entering your reservoir combined with higher reservoir temperatures.
Keeping your reservoir cooler, either with a reservoir chiller or by keeping your room cool, will make a huge difference. Covering your reservoir with either a lid or black and white plastic, white side facing up, will keep the light out.
You can also add hydrogen peroxide to your nutrient solution at each reservoir change to help eliminate the sludge. Cleaning your reservoir between crops is important to keep the sludge from returning. You can use a diluted bleach solution of one part bleach to five parts water, or there are many food-grade industrial cleaners available that will do the trick.
A measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. We recommend that you keep your solution at about 6.5 because that is the point at which the nutrients are the most soluble.
PPM is very easily defined as Parts Per Million and can be used as the measurement of a number of different things. More commonly in the hydroponics world, this measurement is used to measure the amount of Total Dissolved Solids in your nutrient solution or how much CO2 is in your atmosphere.