The Pros and Cons of Using a Marine Aquarium Cover Glass

saltwater aquarium coverTo put a lid on it or not to put a lid on it, that is the question!

Okay, with profuse apologies to the Melancholy Dane, the point I’d like to mull over in today’s post is whether it’s a good idea to use cover glasses on marine aquariums—you know, those oft-hinged glass or acrylic lids that are available in various dimensions to fit tightly atop aquariums of different sizes.

As with so many aspects of the marine aquarium hobby, there’s no all-encompassing right or wrong answer to this question. Suffice it to say that cover glasses may be appropriate in some circumstances but totally inappropriate in others.

To determine what’s best for your system, consider these cover glass pros and cons:


  • Having a cover glass in place reduces evaporation, which in turn can reduce the size and frequency of freshwater top-offs and helps lower the humidity in the room housing the aquarium.
  • Fish prone to jumping or slithering out of a tank are kept in the aquarium where they belong. Some fish, such as eels, and even certain invertebrates, such as octopuses, are such good escape artists that a tight-fitting lid is a must when keeping them. However, for many fish species, there are alternatives to glass/acrylic lids that may do the same job, e.g., covers made of some type of mesh or screening material or plastic egg crate.
  • The light fixture is better protected from splashes and corrosive salt spray. This is especially important if the fixture isn’t equipped with protective lenses to shield the bulbs/tubes.
  • Salt creep doesn’t advance as far with a cover glass in place, though it will still build up on the cover itself.


  • A cover glass can reduce vital gas exchange at the water surface (think of the interface between the water surface and the air above it as the system’s “lungs”). Having an open sump and providing vigorous protein skimming can help compensate for this.
  • Lime deposits and salt creep on a glass/acrylic cover reduce light penetration, which is detrimental to photosynthetic invertebrates. So, regular cleaning of the cover glass (another maintenance task) is essential to prevent this buildup.
  • A tight-fitting cover tends to trap heat, which can lead to unacceptably high water temperatures, especially in warmer seasons.
  • Evaporation decreases. Yep, this can be a con as well as a pro because it also means there’s less evaporative cooling going on, further contributing to heat buildup. A slow rate of evaporation can also be a con for reefkeepers who drip kalkwasser to supplement calcium and alkalinity since saturated kalkwasser is typically used for freshwater top-offs.

What have I missed?
I’m sure I’ve overlooked some pros and/or cons to cover glass use. If you can think of any to add, please let us know in the comment section below.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Dave Malet says

    Con: With glass tops it will defuse some of the light spectrum that may be needed for the Aquarium.With not only the glass but with alga and calcium build up.

  2. Paul Baldassano says

    A con is that it is a pain to feed the fish or do anything on the tank like removing an invasive manta ray that may have come in on a piece of live rock. I like to just walk by my tank and throw in some food and my fish like to respond by splashing me. I did add a cover once, but shortly after, my corals died. I am not sure why but it was made out of plywood which may have had something to do with it.

    • Jeff Kurtz says

      I could have warned you about that, Paul! You need to use transparent plywood in such circumstances!

  3. I have a few fish that like trying to jump out of the tank. However, if I add a perplex sheet to the top of my tank I could limit the gas exchange between the water and the air. Would it be better to drill a few holes to the top on order to let the air out without letting out the fish as well?

    • Hi Stephen! To get around the issue of reduced gas exchange while still keeping the fish in their place, some folks cover their tanks with plastic egg crate material (though it does decrease the light somewhat) or clear mesh in a window-screening frame.

  4. You wrote:
    “A cover glass can reduce vital gas exchange at the water surface”

    Do you have evidence that this occurs or is it just something that seems like it would be the case?

    I’m not trying to be snarky – I’m new to aquariums and it’s an honest question.

    • Jeff Kurtz says

      Hi D! No snark taken. Your question is a good one and perfectly valid. I’m not sure I can point you in the direction of any data to support this idea, however. My statement was based on personal experience with, and other hobbyists’ accounts of, tightly covered tanks (without a sump) that developed issues with low pH levels that then improved after the cover was removed, suggesting improvement in C02/02 exchange at the water surface with the cover off.

      I may incorporate your question into a future “Salty Q&A” post because I think it raises an interesting point: How often do we hobbyists accept “common knowledge” at face value without digging any further.

  5. I made a custom clear mesh screen lid, I wasn’t a fan of the glass tops building up with salt and with our hot one day freezing the next day temps I was always fighting the temperature changes with the lid on. With the lid off and screen mesh on I have a better gas exchange but I’m topping off my water atbleadt twice a week and with cooler temps outside I have noticed a huge change in humidity inside but I have a dehumidifier that helped

  6. Of course there’s a right or wrong answer.

    Right: Protect your fish with a full lid/cover.

    Wrong: Not giving a crap about killing yet another fish even knowing the despicable rate of wild and captive deaths because of this hobby.

    See that wasn’t hard; was it?

  7. There is considerable work on developments toward sequestering CO2 via algae growth in coal flue gas power plants in the energy sector.

    So in a marine aquarium with algae growth problems, maybe putting a cover canopy would limit CO2 levels in the aquarium … Thus indirectly limit algae problems.

    I have a twenty gallons FOWLR with RO makeup water. Would a cover curb algae blooms?

  8. my name is les i do not have live corals i do have anemones and fish i like glass tops i live in florida a/c is a must 90 percent of the year an open top would have massive evaperation.

  9. I would imagine it’s like a house in winter. If you’re in a 12×12 room with all the windows and doors shut by yourself you’re fine. Even in a modern well sealed house. Now add 5 people to the room and now you are starting to get that stuffy air feeling. So you open a window. Viola not stuffy anymore. Add 5 more people probably need to open another window. Now imagine the windows were in the ceiling. You would have even better ventilation due to heat rising and cold settling. That’s where the openings for the filter, cords etc. are in the glass cover. With temperature differential and O2/CO2 wanting to be in equilibrium and other chemistry stuff I don’t know or remember, unless there are no openings at all in the top I’m sure there is more than enough exchange between the air under the glass and the room air except maybe in the most overstocked of tanks. Also if it took longer for the CO2 to “blow” off in the morning, assuming photosynthesis is occurring, keeping Ph lower. Wouldn’t the same hold true for O2 at night keeping Ph higher longer? Seems like it would be a wash to me.

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