FRequency of water changes and problems associated with water changes

I have my 55 gal. reef tank about 8 months now. It was doing very well until lately

I made a water change and I am assuming I took too much water and I must have changed the bacteria. So I lost 2 fish and my anemones look like they are in shock. But, from reading I realize I am not adding any additives which will help my tank.
This is the first time I have heard of Kalkwasser. I have been buying from the same supplier for months and he has never mentioned it. My question is what tests should be performed other than ph,nitrates, ammonia and phosphates.

These are the only tests I do.

That should be fine. Once you add kalkwasser you may want to test for calcium but its not absolutely necessary.

Kalkwasser is mainly for coraline algae and stony corals so I doubt if it had anything to do with the loss of your anemones or fish. When you do water changes the absolute maximum is 50% and thats only to correct a severe problem (algae) or extremely bad water. An average wc should be around 15% ideally done monthly. Also you have to make sure that the temp & salinity are identical or very close.

Problems happen if one doesn't do a wc for a while and the animals get used to that environment (however bad it may be) then the owner does a significant wc thereby shocking the animals. I'd say thats what happened in your case. So just be more careful next time and I really don't advocate using additives. The best way to introduce additives in the tank is with a high quality salt mix done monthly or every 2 months.

I recommend to be consistent with the wc whatever schedule you use.