Water troubles

[Josho]

So, I'm new to the hobby but I've been researching quite a bit. I'm building a 29 gal. Sand, dragon rock, driftwood, and planted with java fern and anubias (16 plants total), as well as java moss and guppy grass. I intend to keep cherry shrimp (already have 30+ in a smaller tank), 10-12 neon tetras, 6 pygmy corys, a clown pleco, and 3-4 nerites. The tank is already cycled and planted but no livestock yet. Keeping temp at 78 and running Aquarium co-op's sponge filter. My issue has been water parameters. I started out just using tap. Out of the tap, it runs 6dKH, 8dGH, and pH 7.8-8.0. I wanted to lower my pH so I got the Buddy+ RODI kit and started doing 30% water changes every week while it was cycling. Apparently I did one too many because my pH crashed down to 6.2. I've managed to get it back up to 6.8-7.0 by doing mixed water changes with RO and tap. My concern is my kH. Current water conditions are pH 7.0, 7 dGH, and 2 dKH. (Aslo, 0 NH3, 0 nitrites, 20 ppm nitrates - adding NH4CI daily until stocked) I tried adding some sodium carbonate in the form of API's pH UP, and that got my pH up to 7 but had no affect on my KH. I'm worried about my plants and pH swings with that low of KH. Am I right to be concerned and if so what should I try differently?

[colo3000]

I had lower KH in my tank so I decided to buy a bag of crushed coral from Petco and put some in a mesh bag. I stuck the mesh bag in my HOB filter and it's been slowly increasing the KH in my tank. You could always use crushed coral as a part of your substrate or possibly even stick a mesh bag in your tank. May be beneficial to test your RODI water's pH just to get an idea if you haven't already.

[Schwack]

What made you decide to tinker with the pH? The only fish that might not love your more alkaline water are the tetra, but they're adaptable little fish. I'd second @colo3000's recommendation to put a mesh baggy with some crushed coral in there and let things settle on their own. Plants and snails ought to be fine with all the wild swings, I've never noticed any die offs when mucking about with water parameters in such dramatic fashion.

If it were my tank, I'd ditch the RODI water and use the stuff with higher kH right out of the tap. Make things as easy on yourself as possible, it's much easier to keep up with maintenance when it's not a huge PITA every time you need to perform a water change.

[Andy's Fish Den]

I wouldn't mess with using RO water in your tank. I have hard water that is at 10 or higher on both KH and GH. Neons are pretty hardy, adaptable fish. If you get them from a LFS that is somewhat local, they probably have water conditions that are very similar to yours. It is very easy to crash your pH using RO water if you're not careful. The only time I use RO water is when I am trying to breed certain fish, and in my caridina shrimp tanks that has been remineralized. 

[Josho]

Thanks everyone for the advice. Sounds like I was just overthinking things. All the care guides I read on neons and corys said they prefer soft and slightly acidic water. I’m a bit of a perfectionist so I was trying to get to those ideal conditions. I’ll do the crushed coral and ditch the RO and stick with tap. Thanks again.