Upgrade to a 200l reservoir. The bigger the res, the less the pH swings.
Don't add so much nutrient. You don't need to follow bottle instructions to the letter. Anything, and I mean anything, you buy to place in your hydroponic setup, use at half strength to begin with. A, so you don't nuke anything and B, they are just trying to sell you more of it!
If you make a fresh solution, and then you come back and find that solution has increased in EC, your plants are using more water than nutes.. so top off with water. If it's the other way, your solution is too weak and top off with a slightly stronger solution.