It's a bit more risky but it can be done. My first hydro attempt was an experiment to see how cheaply I could build my own system and how many rules I could break. I assumed that the tap water, medium and setup were ph neutral and that the cheap nutrients would slightly lower it. It wasn't until 4 or 5 weeks into flowering that I became curious and tested the ph with an aquarium test kit. It indicated it was around 6. I actually added a tiny amount of bi carb dust to bring it to around 6.5 as that is what the nutrient I used instructed. I only tested the new water being added but not the small reservoir.
I do consider myself very lucky that the harvest worked out extremely well!
Here is a picture of some of her very early in flowering and prior to any ph testing or adjustments. Lighting - 48watt CFL, 5000K.