There are several problem issues in your first post. First let me say that I have not heard of nitrite or nitrate being problematic with crushed coral so I would discount that.
Do you have the test numbers? Nitrates being "high" can mean different things. Nitrite too, although nitrite should always be zero in an established tank, same as ammonia. But please give us the numbers, and what is the test kit?
Have you tested your tap water for ammonia, nitrite and nitrate? You should, as any of these can be present and this is worth knowing so they can be dealt with at water changes.
When I have answers to the above, we can consider that issue. Now to the other problems.
Your mix of fish is not compatible in terms of water requirements (pH and hardness). Mollies are livebearers and must have hard basic water; the coral is fine for them. Some tetra can manage in this, many cannot. And rummynose is one that absolutely will not live long in hard water. Please check our profile of this fish, second tab from the left in the blue bar across the top of the page, or click on names when they are shaded in posts, example: Brilliant Rummy Nose Tetra. The info in the profile will explain this.
Pleco. If this is the common pleco, it attains over 12 inches, produces an enormous amount of waste, and is not suitable for such a (relatively) small tank, even if small now. Again, check the profile (click on shaded name).
Salt. This would be OK if only Mollies were in the tank, but not for pleco and tetra. To explain, I will copy my comments from another post:
Salt is detrimental to freshwater fish and plants in varying degrees. To understand why, we must understand what salt does in water.I will provide what advice I can when I know the numbers previously requested. I would also suggest you consider how you want this tank to be in terms of fish.
Salt makes the water more dense than the same water without salt. The aquarium contains water. The bodies of fish and plant leaves also contain water [just as we do--we are, what is it, 70-some percent water?]. The water in the aquarium and the water in the fish/plant are separated by a semi-permeable layer which is the cell. Water can pass through this cell. When either body of water is more dense, the other less-dense body of water will pass through the membrane to equalize the water on both sides.
Water is constantly passing through the cells of fish by osmosis in an attempt to equate the water inside the fish (which is more dense) with the water in the aquarium. Put another way, the aquarium water is diluting the fish's body water until they are equal. Freshwater fish regularly excrete this water through respiration and urination. This is the issue behind pH differences as well as salt and other substances. It increases the fish's work--the kidney is used in the case of salt--which also increases the fish's stress in order to maintain their internal stability. Also, the fish tends to produce more mucus especially in the gills; the reason now seems to be due to the irritant property of salt--the fish is trying to get away from it.
I have an interesting measurement for fish. Dr. Stanley Weitzman, who is Emeritus Research Scientist at the Department of Ichthyology of the Smithsonian Institute in Washington and an acknowledged authority on characoid fishes, writes that 100 ppm of salt is the maximum for characins, and there are several species that show considerable stress leading to death at 60 ppm. 100 ppm is equal to .38 of one gram of salt per gallon of water. One level teaspoon holds six grams of salt, so 1 tsp of salt per gallon equates to more than 15 times the tolerable amount. Livebearers have a higher tolerance (mollies sometimes exist in brackish water) so the salt may be safe for them.
Plants: when salt is added to the aquarium water, the water inside the plant cells is less dense so it escapes through the cells. The result is that the plant literally dries out, and will wilt. I've so far been unable to find a measurement of how much salt will be detrimental to plants; all authorities I have found do note that some species are more sensitive than others, and all recommend no salt in planted aquaria.