Whoops, screwed that up last night (must have been tired). Plants take up CO2, which is acidic (NOT give off). Nevertheless, the addition of live plants will change the equilibrium between the acids and bases and I am not surprised that the pH changed at least somewhat.
Again, however, the change in pH isn't really a large thing. If you look at the definition of pH, pH = - base 10 logarithm of concentration of H+ ions, the change from a pH of 7.5 to 7.0 is a change from 0.000 000 032 moles of H+ per liter to 0.000 000 100 moles of H+ per liter. Not a very large change at all -- we're talking very small quantities of acid here. Using the pH converts brings these small numbers into something that is easier to look at, but a lot of times it is lost just how small a change a pH from 7.5 to 7 is.
And on this note, I'd like to say something about your comment that "I thought 7.5 was a little high" -- your comment is a least a little misguided. Too many people have this notion that 7.0 is perfect water, and that anything but 7.0 isn't optimum. This couldn't be farther from the truth. Firstly, fish live in native waters that have a wide variety of pH: there are fish that live in very acidic streams in the Amazon and orient that have pH's below 4.0, and the African Rift lakes have regions with pH's about 10.0. If there is any "optimum", it would the pH of the fish's natural habitat. However, even given that fact, most fish are exceptionally adaptable and can live very long happy healthy lives outside of the pH of their natural habitat. The main goal is a non-fluctuating pH. If you think about it, fish live in pH environments that change a lot. During the day, as the sun comes up and warms the water and the aquatic plants change in activity, the pH of bodies of water can vary over 1 or even 2 pH units. Also, the runoff during fast rains almost always isn't the same pH as the body of water. Fish are adaptable, you don't have to control your pH to a specific value, like 7.0. So long as the water coming out of your tap is good, the fish will be happy. Only if you are going to try to keep some very rare species or wild-caught species or try to breed your fish, then you may have to start to worry about changing the water parameters. But, most fish keepers don't do any of those things, and end up doing more harm than good trying to find that "optimum".
So, finally, that comes to the issue of your KH. Again, like I said above, I'd test both the pH and the KH everyday to see what the trend is. Your pH will not crash until the KH reaches zero. The equilibrium between the acids in the water and the buffering is complicated, so as the KH gets lower, the pH will go down a little, but not enough to be detectable by home test kits. You'd need some lab-grade expensive equipment to measure the change (the change will only be in the thousandths digit, maybe the hundredths as the KH gets to 1 degree or less).
Finally, you might want to consider testing the pH of your tap water, too. Take a glass of it and let it sit out overnight. You want to let it sit out overnight because the water company will dissolve gases into their water, and you want to let all that diffuse away so that you are testing the water that is similar to the fishtank. I suggest testing the tap water because sometimes, at different times of the year, the water from the water company is different, too.
. It is already looking bad.
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