Blue Acara... personal knowledge?

A friend of mine have them in several tanks. The smallest being 144 Liter (a single pair) and the biggest being 560 liter (a group of fish with specimens in all of sizes). In both tanks the biggest one is somewhere between 10 and 12 cm/4 and 5 inches long. The females tend to be a little smaller than the males, but not by as much as is often seen in new world cichlids.

Contrary to popular belief, the size of the tank is not a limiting factor when it comes to the end size of a fish. Only the amount of waste products in the water decides this. In clean water, the fish will grow to their maximum size. In a smaller tank the waste products do build up faster, which means you have to change water more often.

Four inches is what you can expect. In my sister's back yard there is a canal (Florida) where you can find 6" ones. They have a neat lavender color you don't see often in this fish. Although it fades in the aquarium, there is a definite difference in the ones from her canal.

You probably don't need a 75 for a pair, but they should get a little larger in a larger tank. Adding a wet/dry and a large sump would help even more.

While it is true that the limiting factor to fish size is the pollutants in the water, the practical way to achieve this in the aquarium is dilution. In other words, the bigger the aquarium or system, the more diluted the inimical substances become. Achieving a close to open system in a small container is possible if new water is being constantly added.