What is the smallest saltwater tank you can have without risk to the mini eco-system?

Question

my home is small, not a lot of room but i want to make sure i keep the fish health

Answer 1

Honestly, the best answer I can give you is to get the biggest tank you can afford with the space you have.  This is doubly true if this is to be one of your first attempts at a salt water tank.  Everything is more difficult to maintain in a small tank; salinity, temperature, ph, nitrogen cycle, etc.  

If you end up with something like a 20 gallon tank (I consider anything smaller than 50 gallons to be a "nano-tank" when dealing with salt water), at the very least get some tougher fish that can handle fluctuations better.  Things like damsels and some of the gobies will do well in smaller tanks and they can handle the occasional spike in tank parameters.  They're also inexpensive enough that you won't be out more than ten bucks per fish if things do crash.

When you do start introducing fish, do it slowly!  Get one hardy fish (I'd recommend a chromis for this as you'll have fewer troubles with territoriality issues than if you use a damsel fish), and let him be for a couple months.  Get yourself used to all the work involved in a saltwater tank and go through at least one water change without crashing your tank; then think about getting some other fish.

I really don't recommend anything smaller than 50 gallons for a first time, but I understand the constraints you're up against as well.  Best of luck to you!