Again relax. The 5.3 is way too low and can cause problems. I am most comfortable between 5.6 and 6.2. It is fine to go up to 6.5 if you are trying to get some calcium uptake for a problem.
There are multiple things to talk about here and I really don't know what your fancy is as far as how you want to grow and what nutes you are comfortable with. I have never used Superthrive. But you are way too early to be using that stuff yet.
One thing a lot of Hydro growers do not understand is the res should cycle up and down. If it doesn't you will have poor nute absorption. the nutes come in at different rates at different PPMs. I will dump a chart below but there are nicer looking ones out there. What you will notice is that there is no perfect Ph. At any spot something is not getting absorbed well. One way around this is to add a bunch of something so even though it doesn't absorb well at this Ph there is a lot of it. Another way is to let the res cycle around and let the plant take up what it can (that's what I do).
There are people who try to maintain it very tight... I don't... I try to give the plants the opportunity to absorb them naturally at the right Ph. That said as you read in my other rant once I get a good tea in there it stabilizes and needs no help. I have gone over many weeks before just topping off and never adjusting a thing. And that was during vigorous growth.
That said the Ph should be changing over the grow for the reasons I stated above and I will put another chart below to explain this. The nutes you want in Veg are not the same ratios you want in transition or in bloom or in late stage shut down. Since those change the ideal Ph you want should change with it. People who grow lettuce try to keep it around 6.2 and a good cannabis growers should do the same during veg. but then in bloom you want other nutes so the Ph should be adjusted to the range for better absorption of those nutes.
Ph up and Down can cause nute problems and eventually if you have put a ton of it in there it will stop working. You can read that on the label. Never trust the PPM reading in the res until it has been mixed up in the res for at least 45 min. top off buckets you can measure within 5 min but the res has a lot of factors and when you dump in to off things will adjust.
What you should
NOT do is add anything while it is in the good range unless you are trying to force it to a specific Ph to solve a nute deficiency problem. So if you measure it at 5.6 in the morning and the afternoon you measure 5.9 that is fast but probably doesn't mean anything important. Most likely it will stop moving up and stabilize around there or maybe up at 6.0. What you don't do is start adding down because you think it is going up too fast. When it goes over the top limit you can add a little to bring it back to 6.4. Then what I do is take the time to slowly bring it back to 5.4 over many hours maybe 2 days. And then let it raise itself again. After about 2 cycles of this it will stabilize and need no more adjustments.
What I am saying is you should not be adding UP or DOWN when you are anywhere between 5.4 and 6.4 That is the good range and it wants to fluctuate up and down to get the popper nute absorption. People waste a ton of money and stress the plant significantly trying to maintain a small range that is unhealthy for the plant to begin with. you let it be and it will be fine and likely you will be surprised to see that tomorrow it goes back down to 5.8.
