The most recent set of experiments sent to the ISS included 32 fish which are going to be studied in a space aquarium called the Aquatic Habitat. On Earth, an air pump pushes air which flows out of an airstone and travels, in the form of bubbles, up to the surface of the tank. What happens in space? How do they aerate the Aquatic Habitat?
I'm not that good at biology/practical things. If anyone disagrees with this answer, let me know in the comments and I'll fix/delete this.
1) Fish: Medaka fishes (Oryzias latipes) are ideal specimens for many reasons. They are transparent, making it easy to view the inner workings of their organs. They also breed quickly and easily in microgravity environments, enabling multi-generation studies. Researchers can take advantage of a variety of genetic modifications to these fish, as well. Finally, scientists already have all of the Medaka genome identified, which makes it easier to recognize any alterations to the fishes’ genes, due to factors like space radiation and its impacts like bone degradation, muscle atrophy, developmental biology.
2) Maintenance: The Aquatic Habit (AQH) is a high-tech aquarium designed to operate in zero gravity. It requires a minimum of maintenance by the crew (it even feeds the fish itself) and can eventually be used to house amphibians as well as fish. It’s not the first such habitat in space. Earlier examples flew on space shuttle missions STS-47, STS-65 and STS-90. However, this is the first to be installed aboard a space station. Sitting in a standard payload rack in the Japanese Experiment Module (JEM), the AQH is sealed, self-contained unit with a closed-water circulatory system.
A tricky (bit funny) part: The astronauts don’t even need to feed the fish the occasional ant’s egg. An automatic system handles that as well. It’s LED lights are programmed to simulate night and day conditions and there is a specimen removal mechanism. Two video cameras in the unit allow the fish to be monitored from the Earth.
AIR-WATER Interface: (This is your answer) Small plastic plates at the upper side of each aquarium use a grid structure to trap a small amount of air which are injected by the crew at the start of an investigation. The design provides a stabilized area for oxygen that will enable fish to "peck" air. It was tested using parabolic flights and prevents the water from escaping into the microgravity environment.
"The special bacteria filter purifies waste materials, such as ammonia, so that we can keep fish for up to 90 days," said Nobuyoshi Fujimoto, an engineer at JAXA. "This capability will make it possible for egg-to-egg breeding aboard station, which means up to three generations may be born in orbit. This would be a first for fish in space."
So, Aeration is the same in space. My thought is that, The AQH aquarium would definitely have the best conditions to aerate the habitat.