In my personal opinion as a landlord (I am not an attorney, this is not legal advice, consult an attorney in your state).
Your lease should include wording of any modifications must have owner's written approval prior to any work authorized and who pays for such work. If not and work was done with out your express written approval and it is in your lease tenant cannot modify unit (electrical, plumbing, etc). If so then it is a breach of your lease.
Speak with your tenants first and give them the option, keep it and pay higher rent and water costs, or remove them at their cost by licensed professional (if modifications were done). Most likely they will choose to keep them and have them sign a new lease with the new provisions. Be prepared with both options written up (new lease w/new terms and quote from licensed professional to return unit in original condition) when you meet with them and have them sign it on the spot.
My biggest concern is the proper ventilation, they're actions are making you liable for a potential fire due to bad ventilation. I would include W/D in my units, sub-meter and charge higher rents and have tenants pay all utilities. It improves your property value and attractiveness for new tenants.
Worst case scenario, you are able to serve a "Notice of Breach of Contract" to tenant (asap) and evict.
Do NOT just wait, address this now, you have to set the correct expectations aka train your tenants.