Questions Acid rain effect?

Because I have been using coco for a while now, I rarely need to use any. I may have to add 1ml to my 24l res's once a fortnight? Very rarely needed.

Something I read today may answer this.

What I didnt know was that when a plant is feeding heavily and using a lot of Nitrogen, it gives off Hydroxyl molecules, which are alkaline. This is responsible for the upwards ph drift we have all seen in veg..... Slowly rising ph along with a falling EC,,,,, THis tells most growers the plant is feeding but few will know exactly why the ph rises.

Likewise, what I didnt know was that when a plant is feeding heavily on Phosphorous, it leeches out hydrogen.
So what?
Well, if I tell you that ph stands for "potential for hydrogen", it may make its significance a little clearer.
This extra Hydrogen lowers ph.

SO the combination of air/CO2 being added plus this extra hydrogen answers for me why your ph has been dropping.
The solution is a res change when this happens since there is no other way of getting rid of the extra H molecules.

This is what I was reading
http://www.an-europe.com/whitepapers/WP%20pH%20Perfect_EN_Feb2012.pdf
This may help

"Why is maintaining a stable pH so difficult?
Three major factors tend to disrupt the stability of the pH in any hydroponic system. Learning to control these influences is essential for a successful harvest.
pH imperfection #1: the pH of the water used to dilute nutrients
Freshly distilled or deionized water has a pH of 7. However, the pH of the water may fall to as low as 5.5 within hours of preparation. This is because water absorbs carbon dioxide (CO2) from the air.
The behavior of tap water is even more complex. It contains dissolved and slightly alkaline calcium and/or magnesium salts. In this case, absorption of CO2 from the air makes predicting the pH even more challenging.
Because the calcium and magnesium salts in most tap waters, not to mention even more chemically complex well and spring waters, create such serious problems, many hydroponic growers, from hobbyists to huge commercial greenhouses, prefer using treated water. Although a number of water treatment systems exist, reverse-osmosis (RO) is considered the most economical. Water obtained from an RO system is almost as good as expensive distilled water.
Another option is to adjust the pH of tap water before using it. This can be done with so-called pH up or pH down additives. However, this task is demanding and often done incorrectly—and what’s worse, the acidic and alkaline chemicals used in these products, and the resulting sudden fluctuations in pH when they are added to the reservoir, can be hard on your plants.
pH imperfection #2: biochemical processes in the nutrient solution
Many pH changes are caused by the nutrients themselves.
Karadjov / pH BUFFERING BREAKTHROUGH
5
The more compounds in the water—measured in parts per million (ppm) or by the nutrient solution’s electroconductivity (EC)—the greater their influence on pH.
For example, the urea used in many fertilizers is broken down by enzymes into one molecule of CO2 (a slightly acidic compound) and two molecules of ammonia (a slightly alkaline compound). This can cause erratic changes in pH.
In addition to urea, any compound containing an amide chemical bond (e.g., the proteinates used in many fertilizers) can, when broken down, affect the pH in unpredictable ways.
Nutrient absorption also leads to changes in pH. When a plant absorbs a lot of potassium ions, it gives out hydrogen ions in return. The result is a net decrease in pH. The situation reverses when the plant absorbs a lot of nitrate ions and gives out hydroxyl ions to compensate, thus increasing the pH (Bar-Yosef, Ganmore-Neumann, Imas, and Kafkafi, 1997; Ryan, P.R. and Delhaize, E., 2001). The higher the rate of nutrient absorption, the more dramatic the change in pH.
pH imperfection #3: the substrate through which the nutrient solution flows
The growing medium (also called the substrate) affects pH as well. For example, coco-based growing media undergo subtle changes during your crop’s life cycle that affect the pH of the nutrient solution. Even baked clay pellets, which are far more stable than coco coir in terms of pH, are less than rock solid in this regard.
In fact, every chemical or biochemical process that goes on in the growing vessel changes the pH of the nutrient solution. Each additional factor drives it further from the sweet spot.
In nature, the volume of surrounding soil—teeming with microbes, humates, and other pH stabilizing agents—does a good job of offsetting pH changes. Natural soils thus act as natural pH buffers. That’s why, in outdoor gardens, where the soil itself contributes to a more stable pH, changes in pH are more gradual than in a hydroponic gardens.1
In hydroponics, however, pH stability is a challenge. It is an intense gardening method where the concentration of nutrients and their absorption rate by plants are much higher than in soil. As a result, chemical and biochemical processes influence the pH to a much higher degree than in natural soils or traditional agriculture. The natural stabilizers and buffers in the nutrient solution, mainly phosphates, are weak, so indoor gardeners have to check the pH of the nutrient solution regularly and adjust it when it goes below or above the sweet spot."