Water changes in a rift valley setup

I would do frequent small water changes, That way you will keep changes in Ph to a minimum.

Steve

pH shock is when there is a large difference. Smaller PWC (10-20%) shouldn't dip the pH enough to seriously stress out the fish.

it should be fine if you do a 10%-15%

just stock smart so your nitrates dont go out of control

nick my tap water is 7, my Ph in the tank 7.8. I have crushed coral substrate and tufa rock.

When I do a 50% water change, untreated except for Prime, Ph in the tank drops to 7.6 or so. A 25% change leaves it unchanged.

Tank size 180L; I do 25%x2 weekly.

We've never had a problem, but we also rarely have to do large changes. We usually change only 10 gallons out of 55 at a time. It would probably be better to do several small changes over a period of time if you needed to do a big change

I think the effect of a large water change will be different on each tank, and its good to know what way your parameters are affected by it, even if you prefer a routine of smaller and more frequent changes; in some situations you may simply have to do an emergency large change. In that situation if your water is chemically buffered from e.g. 7 to 8.2 you might have difficulty balancing the changed water and cause Ph shock.

I go by the principle that it has to be over 7 for a start, up to 8.2 if that's what you can achieve with good buffering, but stable at some point lower than that is better than trying to use the chemicals to increase.

Even when I do a 50% water change it doesn't bother me that my Ph drops by a couple of points.