I would not consider getting another invert until you can set up an RO/DI unit. It is critical to keep nitrAtes below 10ppm (zero is the goal) for them.
You should also be able to maintain certain water parameters such as salinity, temperature, calcium, alkalinity, and magnesium. How do those test right now in your tank?
Shoot for the parameters found here:
Reef Aquarium Water Parameters by Randy Holmes-Farley - Reefkeeping.com
Some good test kits to use are Salifert, API, and ELos.