Here you have some useful tips to help you increase your earnings:

EXPAND YOUR VAULT'S CAPACITY

The first thing you have to do to increase your income is to level up the vault in your hospital. Don't forget to regularly check the vault and level it up until you get the max level. Moreover, check the Research list to run all the investigations related to this said room located on the bottom right side of your building. Remember: you can complete the game without spending real money. If you see a price higher than your money limit, there should be a missed level out there! Read the pending requirements in the vault's dialog window carefully.

UPGRADE YOUR ROOMS 

All the upgrades you make in each room will affect your performance and efficiency, so you need to take care of every one of them so they can be harmonized and balanced and can function properly with all departments' help. You can check the upgrades or expansions available and their functions by tapping over each room.

Also, don't forget about the delivery workers and room, as you may be producing a bunch of supplies, but it will not be useful if you cannot deliver them to the corresponding rooms! You need to check the whole medical process from step one.

Wanna increase your number of patients? Check the rooms that turn clients angry and upgrade them as much as possible. Your prestige will rise, and more patients paying more will come.

DO ALL THE RESEARCH

The House > Microscope icon will be your best ally from now on! All these sections will take some time to be developed, but they will help you expand your veterinary hospital. Check the improvements they provide and choose the research according to your strategy.

CHECK YOUR HIRED DOCTORS AND THEIR ATTRIBUTES

Take into account that your unassigned vets will charge you anyway, so if you are not interested in some, review your list of vets and consider firing them if they are not useful in these moments. This action will reduce your expenses and help you continue with the game.