Does a Fish Aquarium Lid Need Holes?

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A good but common question I’ve encountered in my years of fishkeeping is whether or not additional holes in the lid are needed for air exchange.

Contrary to popular belief, an aquarium lid does not need additional air holes. As long as the cords and airlines have suitable holes to fit through, a tight-fitting and well-sealed lid is most beneficial.

As long as there is proper agitation at the surface of the water, gas exchange will occur via the circulation of the pumps and filtration equipment. Daily opening of the lid for feeding and maintenance will supply all of the needed fresh oxygen and allow the toxic gasses to dissipate.

Some lids come with plastic strips that allow specific areas for space to be cut to accommodate cords, airlines, and other equipment. Others have pre-determined plastic punch-outs. These are best when cut minimally to keep the tightest seal possible.

A tight-fitting lid assures several things:

It keeps the right things in.

It ensures a slower rate of evaporation. This is especially important for tanks kept at higher temperatures.
It minimizes salt creep in marine, hard water, or brackish water aquariums.
It insulates against heat loss. This will ensure efficient use of electricity and help keep the temperature within the tank more stable.

It also keeps fish from jumping out. Fish are very good at finding the tiniest of holes and escaping through them, and this is especially true of nocturnal fish.

It keeps the wrong things out.

A tight-fitting lid keeps other pets from investigating the aquarium and possibly harming the fish.

It also helps keep airborne and other contaminants from getting into the tank.
It prevents water from splashing outside the tank.
It keeps the aquarium odor contained and prevents unpleasant smells from escaping.

How the water is actually oxygenated

The aeration in an aquarium is actually caused by the amount of surface area that is agitated. This exchanges gases at the surface of the water. Oxygen is also injected into the water column of the aquarium from air pumps, powerhead intakes, and airlines.

Tanks that are aquascaped with living plants also contribute a lot of oxygen to the tank environment. A well-planted tank with proper lighting will soon begin to release tiny bubbles of oxygen on its own. These rise through the water and collect above the surface. A well-circulated current of water will keep the surface attracting the oxygen and circulate it properly throughout the tank.

Another way that oxygen enters the water is when it’s directly injected into the water currents. Air stones or other diffusers will split the air into tiny bubbles and disperse them further into the water. The rising of the bubbles, especially the ones in the lift of an under-gravel filter, will ensure any stagnant water that settles in the bottom will be brought to the surface and aerated.

Circumstances that may hinder aeration

Oxygen content is higher in low-temperature waters. Some specialized setups, such as those for Discus or for breeding surface-breathing anabantoids like Gourami or Bettas, require temperatures in the mid-80s and a slow water current. Close attention should be paid to keeping a one-inch air gap between the top of the tank and the surface of the water. This will ensure that there is an adequate pocket of air in which to keep the oxygen readily diffusing into the water.

A really heavily aquascaped aquarium can also pose a few problems. Everyone knows that plants produce oxygen during daytime photosynthesis, but at night without light, they will actually remove oxygen from the water.

If these tanks are overpopulated with too many fish, those fish could very well suffer from a lack of oxygen at night. However, this is usually not a big issue as such specialized aquariums often require multiple feedings and daily maintenance.

An exception to this rule is the tightly sealed and heavily planted aquatic garden. They don’t get opened often and are usually getting a steady input of carbon dioxide gas to keep the plants growing as quickly as possible. The chemistry of the water can get a bit unstable, become oxygen-deficient and having a crash in pH, but hobbyists who attempt this generally already have an advanced understanding of aquarium chemistry.

Types of Lids: Pros and Cons

Glass
Glass is cheap, easily cut at any local hardware store, and can be fit to form a tight seal. Glass doesn’t scratch like plexiglass, and it allows the best penetration of light. Plastic handles and hinges can be readily purchased for installation as well. A glass lid holds the heat, water, and salt creep much better than plastic is capable of doing. However, the glass does tend to be fragile and can be easily broken if not handled carefully.

Plexiglass
Plexiglass tends to be a bit more expensive and less readily available to customize like standard glass. It scratches easily, but it otherwise is very durable. Plexiglass insulates much better than glass, but it has a tendency to eventually warp, which is not ideal when it comes to setup expenses. It is also not the best choice for plant tanks or reef tanks with coral as the proper light spectrum for photosynthesis doesn’t penetrate as efficiently through plexiglass as it does through standard glass.

Prepackaged standard aquarium hoods
These are the most frequently used lids for aquarium tops. These combine the benefits of both glass and plastic and are aesthetically pleasing, typically readily available, and also quite reasonably priced.