Increasing water hardness with Texas holey rock

[minyip]

Hello. I'm wanting to add a piece of Texas holey rock to my fish tank to increase the water hardness.
My current water sits at 7.4 pH straight from the tap but I'm finding it drops to 6.8-6.4 in the tank within two or three days after a 50% water change.
My current water hardness is around 2 dGH or 36ppm.

Will adding a 2kg holey rock to the tank bring the pH up too suddenly and stress the fish? Will it make the pH too high or keep it steady around 7.4?
How much will it increase the water hardness? Will it be enough to buffer my pH?

Basically I want to get to the point where I only need to do a water change once a week if possible.

My tank is 127L (33 gallons) and currently has 3 Rummy-nose tetras (don't worry I do plan to get more. I had 6 but the pH suddenly plummeted and I lost 3 to shock) and 5 zebra danios. I plan to eventually get a bristlenose and some nerite snails. Maybe some bigger centerpiece fish like rams or honey gourami. Haven't figured it all out yet.

Not sure if relevant, but my tank is newly cycled. Only been fully cycled for about two weeks. Ammonia and nitrites are at zero. I think the nitrites are just using up the small amount of minerals in my water too quickly and causing the pH drops.

Any insights would be appreciated

[Aiken Drum]

Your pH drop is probably just what your tap water is. Measuring pH straight from the tap isnt measuring your tapwater pH as it hasnt had chance to degas yet. Put some tapwater in a container for a day, run an airstone if you can, then measure the pH.

Adding texas holey rock will increase GH and KH which in turn will probably raise your pH. Only way to know by how much is to give it a go, especially as we dont know your tap waters baseline pH. Personally i would try some cuttlefish bone or crushed coral in your filter to see if that has the desired effect before dropping a large piece of limestone in the tank.

The likelihood is that you dont need to do anything though. Fish are tolerant to a wide range of water parameters, and if you obtained your fish locally they will have been born and raised in similar water to what is coming out of your tap. Regular water changes should be enough to replenish KH, and the fish could have died because they werent properly acclimated.

[Fishlegend]

Holy rock is a really expenssive fix for this issue... id suggest measuring ph like drum said after its released its gasses.