Aquarium Cleaning Guide

Quick answer: a Planet Zoo 2 aquarium is cleaned by two separate systems, and fixing one does not fix the other. Water Cleanliness comes from the filtration network — filters, power, and a Filtration Rate that meets demand. Tank Cleanliness is done by hand: keepers dive through the Aquarium Staff Gate and clean the tank and windows. A third value, barrier Condition, belongs to mechanics and is what produces cracks and leaks.

The three jobs

Meter	Who fixes it	How	What happens when it drops
Water Cleanliness	Filtration network	Filtration Rate ≥ demand	Dirty water: disease risk, poor visibility, welfare loss
Tank Cleanliness	Keepers	Dive in through the Aquarium Staff Gate and clean	Disease risk; decays much faster while water is dirty
Barrier Condition	Mechanics	Service the aquarium barriers	Cracks and leaks, fleeing guests, an inspector fine

Once you know which of the three is down, the fix is obvious. The mistake is treating them as one "the aquarium is dirty" problem.

1. Water cleanliness — the filtration network

The network produces a Filtration Rate that has to meet the tank's demand.
Demand rises with water volume and animal count. Adding fish without adding filtration is the most common way to drop this meter.
One filter covers roughly 500 m² of water according to community guides (grade C); a big tank needs more than one.
Check the filter is actually connected to the node on the Aquarium Staff Gate — connecting it to a barrier panel is a silent failure, the filter exists but cleans nothing.
The filter needs power. Loss of power stops filtration.

2. Tank cleanliness — the keepers

Keepers enter through the Aquarium Staff Gate, in diving suits if the gate is raised.
They clean the tank interior and the viewing windows.
The gate has to be connected to the path network, or no keeper ever arrives.
Tank cleanliness decays on its own, and much faster when water cleanliness is low — so a blocked filter turns into a staff workload problem too.

3. Barrier condition — the mechanics

Mechanics service barriers; aquarium barriers are included.
Low condition produces cracks and leaks. Guests flee a leaking tank.
If an inspector finds one, the inspection ends there and you get a fine.

A routine that avoids most problems

Check filtration first. Water cleanliness cascades into tank cleanliness, so most "dirty aquarium" reports are a filtration problem with a cleaning symptom.
Keep a keeper and a mechanic within walking distance. Long paths delay cleaning and repairs, and both meters keep dropping while staff walk.
Re-check filtration after every expansion or new animal. Demand is recalculated from volume and population.
Repair leaks immediately. Barrier condition also drags guest happiness and inspection results.

Community-reported aquarium issues (leads, grade D)

Tank stays empty after building — usually a normal habitat Staff Gate instead of the Aquarium Staff Gate.
Aquarium Gate crash in the Nordic Fjords tutorial — Chapter 2 career.
Deep-water welfare bug — reports of the blacktip reef shark's welfare resetting to 0 after loading and sticking on "calculating".

These are community leads, not Frontier-confirmed fixes. Check the Under Investigation list before relying on a workaround.

Sources

Frontier Player Guide — Aquariums (grade B: the two cleaning systems, filtration network, barrier condition)
checkpointguides — Planet Zoo 2 aquarium guide and gamefieldguide — aquariums & aquatic animals (grade C: keeper cleaning, filter area, inspector behaviour)