If you think a request prior to them moving in is bad, just wait until they start asking you to change light bulbs, plunge toilets, replace electrical sockets, repair their furniture, buy them groceries, etc.

And Septic is not a big deal as long as it is maintained (you/your tenants must never put any bleach or chemicals down the toilets, tub or sinks). 

He's not the guy you'd call for a leaky toilet, but he can take on a rehab. 

During the process of them moving out, i found out a few things that they left in the apartment. 1. keys were all not given correctly, 2. walls painted in orange, blue and green colors, holes in living room, and hallways, toilet clogged with rent receipts, garbage everywhere especially in 2 rooms, and so forth.

You gotta put toilet, shower, floor, vanity, fixtures, paint etc. very meticulous.

In theory.Most of the time it will not actually pan out, in practice, for various reasons.

If you don't pay for water as a landlord, what do you do if the renters cause high water bills (ex:  don't shut off water in winter, or don't report running toilet promptly)?  

Just like in the gold rush the money was made selling the pots and pans to the gold miners.and your results even though you have some leads then you need to figure out how good you are at converting said leads.. are you in a position to actually buy them or are you just trying to tie them up and flip them with only putting a few bucks up.. ???

Does anyone have any referrals when it comes to painters or toilet installers in the city?

Also, each person is more wear and tear on your fixtures, materials, doors, toilets, etc.