Apart from the uncanny resemblance between the Infinity sign and the Pisces Fishes swimming in opposite direction there is a lot in Pisces which makes it a sign of Infinity.
Pisces is the last sign of Zodiac and frequently associated with emancipation or “Moksha”. The yogic cultures define emancipation not becoming one with the universe and getting the ability to experience the entire universe as part of oneself. This is another way of attaining infinity.
Also the Pisces fishes symbol shows two fishes deep down in ocean, swimming nonchalantly. Ocean is always associated with deep mysteries and infinite beauty. The fishes at the bottom of the ocean have all they need in abundance right there. They do not need to come on the ocean surface. This symbolizes a fishes (a living being) swimming calmly in the unbounding water (symbolizing sea of Consciousness).
Efficiency, getting somewhere in a rush, doing things in hurry – all these are required only when there are limitations on resources. However if there are infinite resources available, then there will be no need of above things. Therefore Pisces folks are hardly seen rushing or being in hurry. They tend to behave as if there is some infinite wisdom, infinite consciousness with them.
However different planets in this environment behaves differently. For example Mercury finds its analytical and mental powers pretty much rendered to not much of use hence feels debilitated. (Why Mercury Debilitates in Pisces). But please don’t consider debilitated planets bad.
Then there are other planets which thrives in this environment of infinite resources such as Venus, Jupiter etc.