KH? WTF?

[gregporterhouse]

I am very new to this site so apologies if this post is in the wrong spot.

I recently added a CO2 injection system to my 20 gallon tank and have been supplementing the system with Flourish and Flourish Excel. I have been using Aquasafe to neutralize the water and have a general substrate mixed with some run of the mill tank gravel. I have a T5 bulb and a pH monitoring system.
The dKH in my tank is about 13 and my tap water is at around 10. The pH is around 6.8. GH is about 15.5. Nitrates and nitrites are great.
Anyways I have read literature that says that I want to keep my KH much lower. Why is this? I understand that it is a measurement of the carbonate hardness, but what does that mean? (I am in organic chemistry but feel free to dumb any response down as much as you'd like!) I am hoping to provide an optimal scenario for the plants to do well, however I lost a fish yesterday and I am not sure if it was too much CO2 in the system or a matter regarding the KH. The pH was down to about 6.4 but all of the other fish were fine, and not gasping at the surface.
Anyways what is an optimal KH for maintaining a healthy system? Are there any simple ways that I could combat this high KH? a bit of baking soda with water changes? Is it necessary?
My plants seem to be growing alright except that some of the older leaves on my new ludwigia seem to be forming brown spots that are decaying away. I think it might just be the shock of the transition and altered water quality, but i am not sure.
I would love to hear any input so please help me with any advice you could give. THX a lot.

 

[Lupin]

You don't need to lower the KH. Keep it where it is.

What exactly are your nitrite and nitrate? This part is irrelevant right now but I find it concerning when you comment both as "great". How "great" exactly are both?

When you lost the fish, what species was it? Any unusual symptoms aside from not gasping at the surface? Was it newly bought? If so, how did you acclimate it? If not, when did you buy it?

 

[kzimmerman]

we really need more tank specs to help you out. Size, lighting, water changes, plants, stocking and stocking density, actual ammonia, nitrite and nitrate readings, how long the tank has been set up.
I have a 55 with very hard water. Both the KH and GH are high, with the pH at about 8. Perfect African Ciclid water, but for some reason I decided to do a planted tank with natives. The cabomba is growing like crazy, the valisneria is sending out runners, with at least one new plant per runner per week. The java ferns are growing slowly, which is their thing. the only plants that are having trouble is the crypts. They keep getting uprooted by the cats and crayfish. Generally, KH isn't going to harm your fish. But we do need to know what kind of fish you have.

 

[FSM]

One 24" T5HO bulb is only 24 watts. Using CO2 with that little light isn't really worth it IMO.