Should I get the TDS pen? I do have kH and gH liquid test available to me and the kH was at 1. So that is pulling my pH down? So my plan is to do small daily water changes until everything becomes stable. Then increase my schedule to 2 water changes a week and see how that goes because I need to keep the dissolved solids down, right?
Yes, yes, and yes, to all your questions. I think you got it perfectly right.
Start with small daily water changes to slowly alter the water back to your tap. A TDS pen would be great as it's easier to aid you during the process. Depending on your situation, I think anything not above 70-80ppm change downwards or upwards daily is not an issue but you need to reach a point where there isn't much more than 40ppm between your tank and tap at any given time long term. You can do first daily 20% water changes, then slowly increase until you reach a point when you can do large 50-60 and even 70% water changes at once if needs to keep that Ph up and not falling. Keep in mind that when you start slow at 20% daily, it may take weeks until you can rush it up to 50% or more. You have to figure what it takes for your tank to stay stable....It's different for individual tanks. But you don't have to do daily water changes long term, just reach the point when any large water change won't harm your fish at any given time. That's the point when your tap and tank don't differ TDS wise by much at all, a safety level is 40-80ppm even but I'd aim at as low as you can achieve long term.
Your Kh is low and as you thought is part of the problem. Kh is carbonate hardness and it's a buffer which keeps your Ph stable. When Kh gets low as that, your Ph goes all over the place. It drops when water changes are not done, it goes up when water changes are done. You need to get to a point when that doesn't happen and the Ph is linear.
Test your tap water. If you have very low Kh from the tap also, you have two options: Large regular water changes or get some crushed coral, put it in a bag as media and in your filters. Then do at least 50% water change weekly long term but after you slowly start with the small water changes as I initially explained. Or just try to find the balance between keeping the Kh close to your tap and at least 2dhg, and the TDS should always be no more than 40ppm ppm difference(that's my personal target I've found useful over the years) and even a bit bigger margin is acceptable. Don't let it increase over time to a difference in a couple of hundreds for example.
It takes a bit of learning, trial and error but if you keep the stats stable, your fish will be fine long term.
For me that takes one 50% water change weekly on a well stocked tank. So not such a big problem. With less stocked tanks, even less can do. The point is to find your "balance".
