Can I add salt to my tropical tank to make it marine?

Making a marine aquarium requires far more than just adding salt to a freshwater aquarium.\

Please read some books on marine aquarium keeping before you even start.

This will save you much grief, as well as truckloads of money.

Anyhow, using the same water (or the same everything) from the freshwater tropical tank will not save you having to "cycle" the tank for saltwater use. The beneficial bacteria that make biological filtration work in salt water are different species from the bacteria that make it work in fresh water. The freshwater ones can't live in salt water, and vice versa.

So when you change to salt water, you are starting over as far as biological filtration (the nitrogen cycle) is concerned.

As a parenthetical comment, let me point out that few of these filter bacteria are in the bulk water at any given time. They are attached to surfaces in the aquarium system, such as the filter media and the gravel in the tank.

You really cant just start a marine tank with one year of experience with a tropical tank, you need years of experience with tropical fish and need to know some chemistry

It MUCH more than just adding some salt to tap water

You have to buy other filters, other heaters, living rock, living sand, something to make the water stream at least 3 times harder than in a tropical tank, special lights, ....

Changing a tropical tank to a marine tank costs over 1000$

You have to change the whole tank you need live rock live sand ,You need to do a lot more research before you think about going Marine .

add me to the list of people saying "nope"

It doesn't work like that