75G with pH and KH issues

[DrNic]

I started a similar thread to this one about a month ago but after 7 pages of replies the subject was beginning to drift and I wasn't sure about a solution. As it stands I setup a 75G tank about 2 months ago. After an unfortunate incident with some fish I added I realized that the water parameters for the tank are WAY off. I picked up some Jungle 6 test strips and tested both the tank and my tap here are the results.

Tank
Nitrate: 5ppm
Nitrite: 0
Hardness(GH): 50ppm
Chlorine: 0mg/L
Alkalinity(KH): 0ppm
pH: less than 6.0 (probably around 4.5)

Tap
Nitrate: 5ppm
Nitrite: 0
Hardness(GH): 50ppm
Chlorine: 0.75mg/L
Alkalinity(KH): 5-10ppm
pH: ~6.7

There is clearly a problem with the pH in the tank as well as severely low KH levels. Two solutions were suggested; (1) use baking soda to raise the KH and thus the pH and (2) add crushed coral to the tank (probably in the filter).

I tried using baking soda in the tank which was a temporary fix. The pH definitely rebounded into the 7.0-7.2 range but began to decrease again within days (without a water change) ending back at 4.5 but day 3. I was also concerned about the amount of salt that was generated by adding such a large amount of baking soda to the water.

I haven't tried adding coral yet. From what I've read coral can be hit or miss. I also worry about the residue I hear about it leaving in tanks sometimes.

Is there anything else that I can try to get these parameters back in a reasonable range? Surprisingly enough there are still 2 fish in the tank, a pearl gourami and a single neon tetra and a number of plants that seem to be doing fine. There is some mulm building up in the gravel as well that I hoping would help stabilize the KH but clearly that's not happening.

Any suggestions would be great!

[Lab_Rat]

Epsom salts will harden your water to help give more buffering capacity. When I had soft water and mbuna I dosed both baking soda and epsom salts. With the epsom salts buffering the water I didn't have issues with pH swings. You could also try one of the commercial salt buffers out there designed for RO water.