Without knowing if your water is good quality - acidic or neutral, high iron, etc - and the fact that it serves 20 units, I would probably start planning to replace it.

You'll have to replace the carpet, carpet padding, and acid wash the concrete underneath.Cats can easily DESTROY carpets, windows (scratching), curtains and drapes, doors, and so on with their scratching.

By some hardcore ( almost pure sulfuric acid) drain cleaner...use every time a tenant moves out....and dont get it on you.

I empty the water from the toilet bowls, pour muriatic acid in the bowls, use rubber gloves, Comet powder or some other brand and I make the worse-looking toilets in about 10 minutes and make them look like they just cake out of the box.

If its well water , and the water is acidic , and you have copper pipes , you will get bluish stains in the sink , tub and toilet .

i've heard of boric acid?

I would have all the floors done in white concrete and acid stained for good looks, easy clean up and it will last long. 

It also depends to some degree on your water supply - how acidic it is - and climate,  etc.   

Works well if installed properly.I had a property with pex and either because they were not crimped properly or for whatever reason, there would be a slow leak and the slightly acidic water (well) would eat away at the fittings, causing many leaks and replacements. 

I would acid wash it to clean it up and seal it with a low luster stone sealer.