Just a quick fyi....it is pH and not PH and it is potential for hydrogen.....in other words are the conditions right to allow for the presence of hydrogen and how much. So a higher pH has a higher potential of hydrogen in the water (or other liquid).
Also the levels depend on whether or not you have a planted tank and it looks like you do. The plants require a certain amount of dissolved minerals. Do you use CO2? Do you fertilize? Do you test nitrate and iron levels regularly. Generally between 6-8 degrees of hardness for both (KH and GH)should be adequate for your plants. It would be better to have one test kit that tests specifically for each parameter. Make your changes slowly....if your tank has managed this long, a slow change is better for your fish.
You want to be very careful about adding things to the tank in an effort to adjust things like the pH. It can often cause secondary problems....like adding phosphates to lower the pH will kill your plants. It is also important to monitor all nutrient levels....if the nutrients are too low, the plants will not do well, but the algae will thrive. When the nutrient levels are at the correct level, (and there is adequate CO2 and adequate lighting) the plants will out compete the algae for the nutrients. The plants will grow, the algae will starve. You will still get some algae, but that can be managed by a good cleaning crew (snails, Otos, SAE's etc)
I too use RO water mixed with my tap water (well water we live on a farm) If you are using city water you have to be very careful as they have to put lots of harsh chemicals in it. You can get products like Seachem's Equalibrium to put the minerals back in the RO water. But this will not address the alkalinity, KH, this only affects the GH. To increase alkalinity, you can use baking soda (sodium bicarbonate) I believe the website below talks about this as well. If you are not using CO2 you may not need to increase your alkalinity very much if you are doing regular water changes. I did notice that after I started using CO2, my pH became much more stable with only minimal ups and downs that coincide with the day / night schedule.
I used to test my pH with the regular liquid test kits, but found them to be unreliable in that different manufacturers would give me two different results. I now use a Pinpoint monitor that gives me an accurate reading at all times. Then by testing the KH, I can know how much dissolved carbon is in the water through a chart posted on the following website (and other places). This website has some very good information for planted aquariums, water levels etc. By maintaining appropriate CO2 levels and nutrient levels, I have more than doubled my plant growth and my tanks have dropped in the amount of algae. I used to have to scrape the glass each week to keep it reasonably good looking. It is so much better. For my CO2 I am using Nutra fin Natural plant system. It has a great little bubble ladder that allows for maximum dissipation of the CO2 into the water. It takes a little more monitoring and fussing than an automatic set up, but for smaller tanks it seems to do a nice job. The instructions say the product is meant for 10, 15 and 20 gallon tanks (there are adjustments on the ladder), but I have experimented a little, (with the help of the recipe I found on the website below) and believe you could use this on a larger tank possibly up to a 35 gallon by mixing your own bakers yeast solution at a little stronger rate. (NutraFin uses yeast and sugar to ferment and produce the CO2). There are also instructions on the website below to make your own. I just really like their bubble ladder which you can buy separately with the tubing. Using this system will not give you the lush tropical growth that an automatic system will, but it will improve the growth and health of your tank. You should also be aware that you cannot turn it off at night so you really have to monitor it closely until you get used to what it is doing. At night when the lights go out that plants produce CO2. That along with the continued input of CO2 will naturally cause the pH to drop. You want to watch this closely until you get used to the system that it does not drop too far.
This does seem to cover more than what you were asking for.....but you can not believe what I have been through with my tanks before I figured all this out.....hope it is helpful.
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