ppm and pH question - AN nutes

I'm using Advanced Nutrients three part and Big Bud.

When I clean my reservoir in my flowering room then add nutes to it I find I have to do a lot of pH down adjustments sometimes 2x/day for about 7-10 days. The pH goes from 5.5-6.1 in about 12 hours. The ppm is around 1700-1800. After about a week of constantly topping off the water I decide to just top it off with the same ratio of nutes I added before to get the ppm to where I had it (1800) plus about 50 ppm to take into account for the ppm im getting from adding tap water each day. So at 10+ days the pH is pretty steady and easy to keep at 5.6-5.7 and only adjust 1x/day.

Is this normal for the pH to mellow out after a week? Since my reservoir stays very clean even after 2-3 weeks is it ok to just keep topping off the nutes and raising the ppm back to where it needs to be each week for lets say 4 weeks? Or am I just better off dumping and remixing the nutes back in to the fresh water every 7 days?

Ph will always go up over the week, i use about 5-10ml ph downer everyday, using AN. Replace your rez weekly, u do not want old nutes and plant waste in your rez.

your ph is going to rise and fall just let it go for the ride your plants benefit that way because they take in different nutes at different ph levels " i know u probly know this" my ph goes right upto 6.4 and then back down to 5.3 sometimes. remember your ph is going to rise when the lights are on and drop when the lights are off. i have a 3 litre rez and have a 5litre jug , i add the remainder nutes in about 2-3 days when theres room