Water parameters consistently low KH with high GH

[LewA]

I have a 29 gallon aquarium with Eco-Complete substrate, planted with both an on-back filter as well as a sponge filter. My tank has been setup for about 1 1/2 months with weekly 25% water changes, and seems to be fairly well cycled with consistently 0 nitrites and ammonia, and nitrates maintain between 10 and 25 ppm. I use the Aquarium Coop test strips for daily testing. However, I am consistently getting low KH (40 ppm or less), pH about 6.8, and my water is hard with GH consistently between 150 and 300 ppm. I want to elevate my KH to the moderate range of 80 to 120 ppm, but I am concerned that using crushed coral will also increase the hardness even more. My plants seem to be thriving, as do the fish (after losing my 2 male guppies and 3 otos over the last few weeks. I have minimal algae. So, if I add crushed coral to elevate the KH, will this make problems for my already hard water aquarium? Or, is it best to add the coral to my filter? Thanks!

[ScottieB]

Why raise the KH? If everything is going good, plants are doing well and the fish wont care about it why mess with a good thing? I try not to get caught up chasing numbers but if you really want to increase KH maybe consider a small amount of baking soda in the water you use for water changes. A little bit goes a very long way though and will increase pH as well. 

Also I love the strips but before you mess too much with chemistry you might want to validate that number with another test.

I posted this reference to another thread but it may be helpful here too; https://users.cs.duke.edu/~narten/faq/chemistry.html

[LewA]

Thanks, ScottieB for your thoughts and for the informative reference!