What Do Aquarium Divers Scrub, and How Often?
At the National Aquarium in Baltimore, Maryland, more than 130 staff and volunteers are scuba divers. In big exhibits such as the Blacktip Reef, they work in 60- or 90-minute shifts, mainly scrubbing algae off the fake coral and off the acrylic windows so visitors can see in. They use brushes of different sizes, soft hand mitts to avoid scratching the acrylic, a custom pressure washer for tougher coral, and a vacuum [1].
The aquarium says staff clean habitats every day, often several times a day, and that some big reef tanks get scrub sessions a few times a week [1]. Living animals share the water with the cleaners, so the aquarium introduces a new machine slowly, running it for a little longer each time as the animals get used to it [1].
Why Do Some Tanks Get Drained and Bleached?
Scrubbing does not reach everything, so some small tanks are emptied and disinfected. At the National Aquarium, the 320-gallon jellyfish habitat is drained about once a month and bleached for roughly six hours. Then a chemical called sodium thiosulfate neutralizes the leftover bleach before salt water goes back in [1]. The neutralizing step matters because chlorine, which is good at disinfecting tap water, irritates and harms aquatic animals [2].
Aquariums differ a lot. The same article says some deep cleans happen monthly, some every few months and some once a year [1]. There is no single schedule.
What Did the Otaru Aquarium Notice About Its Yearly Big Clean?
In Japan, the Otaru Aquarium on the coast of Hokkaido used to drain its tanks once a year and clean them with chlorine. The idea was to remove algae, uneaten food and waste so the fish would not get sick. A Hokkaido Shimbun video report about the aquarium carries the headline "Stopped the big cleaning, and the water quality became stable" [3].
That headline is the part we can read. The rest of the report is a video, and I could only read its short written summary, so I cannot tell you exactly what changed or how fish health compared before and after. What the headline does suggest is that a tank wiped completely clean was not automatically a more stable one. To see why, we need the bacteria.
Which Invisible Helpers Keep the Water Safe?
Fish breathe out and pass waste, and food that goes uneaten rots. All of this puts ammonia-type nitrogen waste into the water, which is harmful to fish. Certain bacteria use it as fuel and change it, in two steps, into nitrite and then into nitrate, which is less toxic; this process is called nitrification [4]. At the National Aquarium, bacteria in the filters help break waste down. One filter even holds a honeycomb-like material that gives bacteria a place to live and feed on leftover waste [2].
Now picture a tank that has just been scrubbed with bleach. Many of the bacteria living on the surfaces may be gone, and the fish are still making waste. That is one reasonable way to read the Otaru headline, but it is an explanation, not something the report itself spells out.
How Does a Giant Tank Build the Helpers into Its Filter?
Shikoku Aquarium in Kagawa, Japan, explained its system to a local TV station. Its main tank, "Watatsumi no Kei," holds 650 cubic meters of seawater, about 172,000 US gallons, at around 73°F (23°C). A filter runs around the clock, and all the water in the tank passes through it in about 20 hours. Water first goes through sand to catch particles. Bacteria in the sand break down ammonia. Then ultraviolet light kills germs, a skimmer removes protein, the temperature is adjusted, and the water returns to the top of the tank [5]. That is for one tank; other aquariums use different numbers. If you want a rough feel for the flow, 650 cubic meters in 20 hours works out to about 540 liters (143 US gallons) a minute on average. That is my own division, not a figure the aquarium gave.
So Is Cleaner Always Better?
Not in the simple sense. Divers wipe off the algae that blocks the view. A few small tanks are drained and disinfected. And bacteria living in the filter help keep the water safe every day. Cleaning that kills those bacteria can undo some of that work, which is why aquariums choose how often and how hard to clean for each tank. It also means "spotless" and "healthy" are not quite the same word.
One open question: exactly how Otaru changed its routine, and how other aquariums balance this, are not in the sources I could read. If you visit an aquarium, look at how the staff clean the glass, and ask how they keep the water healthy. You might get a better answer than this page.