ph constantly dropping! please help!

I am running 7 plants from a 15 gallon tank.
Using ro water with sensi cal, canna aqua base, rhizotonic, cannazym, boost and liquid silicon. I don't have to do a lot of initial ph setting because after I have mixed my nutes the ph is at about 6; I adjust to 5.8 and off I go. I am using a wilma system as a flood and drain with clay pebbles and the plants are watered once an hour for 10 mins lights on and once in the middle of the night.
After 24 hours my ph consistently drops to about 5.5 and if I don't adjust it goes down to 5.2 in another 24 hour period!
Should I just keep adjusting every 24 hours or have I got a more deep seeded problem?
Does anyone else use any liquid silicon products? I.e budlink, dyna protekt or rhino skin could the absorbtion of silica cause the ph to drop so rapidly?
Any input would be greatley apreciated thanks!

http://plantcellbiology.masters.grkraj.org/html/Plant_Cellular_Physiology4-Absorption_Of_Mineral_Nutrients.htm this is probably a good place to start. I am fairly certain there must be something related to root respiration and ion exchange taking place.

If you are in hydro I would look into bad algae or bacteria in your water and on your roots. If this is not the case what nutrients are you using at what dosages? I have found that running a higher concentration of nutrients in your res will stabilize the ph a little better because of the buffers in the nutrients themselves. Technically silica can raise your ph so the absorption could lead to a ph drop. Since you have a ph pen I am assuming you have a tds meter as well. If so watch your res and see if tds are rising or falling and get back to me.

Sorry I've been busy. Thanks for the replies, I do not have any algae as far as I can see, I may have bad bacteria I don't know how can I be sure? I'm running canna aqua at a quarter of the recommended dosage I find if I raise the ec I tend to burn my babies. Starting water ec 0.1 reverse osmosis plus advanced sensi cal to ec. 0.3 and then canna aqua a and b to ec 1.0. The ec does not seem to be rising. I could understand how the low concentrations of base nutrients could cause the ph to be less stable but i have always used these nutes at a much lower dose than recomended and have never had my ph drop like this; if anything it always used to rise slightly.
Someone also told me reverse osmosis water is very unstable even with the use of a cal mag supplement so I tried mixing 50/50 tap water and reverse osmosis water to get an initial ec of 0.3 and then add canna aqua a and b to get a otal ec of 1.0; still my ph drops very rapidly. I can only imagine that maybe my initial use of pure reverse osmosis water may have given bad bacteria a chance to colonise my growing medium as there is no chlorine or chloramines in ro water. What can I do about this? I don't want to use hydrogen peroxide or anything that could harm the beneficial fungi that I forget to mention I had installed in my root zone. Maybe if I add some beneficial bacteria ?
again any input would be great and I hope someone can help, if not I fear I may have to take the plunge and get my self some advanced nutrients ph perfect base nutes because I can't keep babying my resovoir.

I forgot to mention Im two weeks in flower and 1 of my ladies is looking very stunted. Could 1 problem plant cause the ph in a re circulating system with a reasonably large tank to fluctuate so much?

I think your nutrient solution is too high. The plants are taking in the water and nutes they need and the leftover solution ends up being more concentrated thus lowering the PH, and Im willing to bet your ppm rises slightly too. I would suggest diluting your solution and letting it naturally rise again before topping off and lowering it again.
Though this is just my opinion.
Good luck.


edit:
Heres a good read on using beneficials in hydro, for root maintenance and protection.
https://www.rollitup.org/dwc-bubbleponics/361430-dwc-root-slime-cure-aka.html

They are showing slight signs of over feeding even though I'm only using 1/4 strength nutes.
I've lowered my ec to 0.9 and I'm gonna get some voodoo juice tomorrow to add some beneficial bacteria. Hopefully that will sort it.