Everything about escape-proofing a bumblebee goby comes down to scale. This is a very small, boldly striped bottom fish that perches on rock and glass, uses its fused pelvic fins like a little suction disc, and can work its way up damp glass above the waterline. A gap you would dismiss as far too small for any fish — the notch around a heater cable, the slot beside a filter — is a doorway at goby size.
Why such a small fish gets out
Bumblebee gobies are territorial perchers rather than open-water swimmers, so they explore the tank's surfaces methodically, including the glass near the top. In their brackish home waters they sit on exposed surfaces at the water's edge, and that instinct carries into an aquarium: a goby may climb, rest half out of the water on the rim or a filter housing, and then topple or wriggle the wrong way. Their size also lets them pass into overflows and filter chambers that would never admit a larger tankmate.
Sealing a tank to a goby's dimensions
Fit a lid that covers the whole top, then cover every cutout with fine mesh — the mesh must be fine, because a standard feeding gap or coarse grill is still wide open to this fish. Add a sponge or mesh guard to the filter intake, block the route into any overflow or sump with mesh the goby cannot pass, and keep the waterline a little below the rim so a climbing fish has a dry margin to retreat from rather than an edge to cross. Check silicone corners and the fit around pipework with a fingertip at goby height: if your fingertip finds a slot, so will the fish.
Spotting trouble before it becomes a loss
Evaporation and top-ups change the maths
Small brackish tanks lose water noticeably between top-ups, and a falling waterline can tempt a keeper to run the tank open or overfill it 'just this once' after a water change — both undo the goby's security. After topping up, confirm the level still sits below the rim with the lid fully closed, and check that floating debris or a displaced thermometer has not wedged the cover up at one side. Take the same care with the little things a goby perches on: suction-cupped heaters, probes and filter outlets all create stepping points toward the top, so arrange them low or accept them as part of the climb and make sure the lid above them is flawless. A perched goby at the surface is normal; a perched goby on the rim is a warning.
Do a headcount at feeding time, when gobies emerge to compete for food — a fish missing from its usual perch deserves an immediate search of the filter chamber, the overflow and the glass above the waterline before anywhere else. During water changes, keep removed rockwork in tank water and glance over each piece, since a perched goby can ride out unnoticed. If numbers keep dropping with no bodies found, assume an equipment route and re-audit intakes rather than blaming tankmates. KinsLe's Lost Pet Alert System lets you flag this goby's climbing habit and tiny size on its profile, so anyone topping up the tank checks the rim and overflow first.