This is a great question Georgi! I am learning about aquaponics and hydroponics so there will be far better answers. Aquaponics deals with the addition of fish, hydroponics is what you are doing, yes? That would make a difference with pH or rather testing and choosing a pH that would work for both your plant selections and the fish.
pH is a major deal for plants (not to mention aquariums and hot tubs). For instance, potatoes love a lower pH than one would think. Most plants thrive at a neutral 7.0 or 6.5 pH. Potatoes need at least a point lower.
One of the major differences between those that are great growers of plants and the rest is knowing the pH preference, needs, of the crops one is growing. For hydroponics that would entail a few 'beds' parallel versus in series to allow for differing pH needs. Hey I am just guessing, grins.
One of the best sources for hydroponics is Jorge Cervante's Cannabis Encyclopedia. Truly one of the best most detailed right down to the nitty gritty chemistry, technology and botany. Only one plant is the star in this book which actually makes the information more understandable and usable. Cannabis loves a bit more alkaline pH. Growing this plant in 6.5 pH makes for a very wimpy plant. Raising the pH to slightly above 7.0 is best for this particular plant. pH is very very important. Anything you can do to be on top of all your plant's needs will be critical in such an artificial system. I am glad you are thinking about pH!!
Gotta get some pH testers...and I'd get at least 2 different ways for testing to check results. Feel free to correct anything I've said that is questionable?