Ideally to maximize oxygenation, the fish tank should have a large exposed surface area, but I find that in the commercial ones the roof is closed. Also is it OK if we keep the roof exposed ? I tried it but it smells awful near the surface of the fish tank.
Nearly any cover you buy or build will allow enough air to circulate. If you're concerned, you can just leave a bit more space between the water surface and the cover, but this really isn't a big deal in practice. And there are some good reasons to keep a tank covered:
- A cover reduces evaporation, which means you don't lose water as quickly. This can be pretty noticeable in warm water systems. Evaporation also leaves mineral deposits behind. In saltwater tanks this quickly leads to 'salt creep', where a thick salt crust forms over everything nearby; even freshwater tanks will start to look a bit gross if you don't keep that clean.
- Many fish can and will jump out of the tank. Nearly any fish the swims near the surface could do this when they're startled. Some, like hatchetfish, actually prey on low-flying insects and are notoriously good at finding small gaps in a hood to jump through: in their case a hood or cover is mandatory.
- Depending on your tank's light, the cover might be necessary to actually support it. If the light doesn't rest on the rim of the tank, you need something to hold it up.
- On a similar note, a cover will keep foreign objects from falling into the tank. If there are young children or pets in your household, this could be a very important consideration. Same thing if you keep any supplies above the tank.
But a cover isn't mandatory in every tank. There are some very attractive open-tank setups, though the ones I've seen tend to have few top-swimming fish in them. It's just a question of knowing the tradeoffs and deciding if it's worth it to you.
My 75 gallon & 30 gallon tropical fish tanks have hood lights but are partially covered with a grated screen because the tops don't fit the entire surface area. If I didn't have them completely covered, the fish could possibly jump out.
On the other hand, I have 40 gallon breeder tank with 4 aquatic frogs, 6 fancy goldfish & 6 bumblebee catfish. The tank is about 3/4 filled with water (not to the top) and I've had the tank for many years and never had a cover on it and never had a problem. But if the water was filled to the top, the frogs could possibly jump out.
It is absolutely ok to keep aquarium completely open and uncovered, if you do not have a specific reason to cover it. There are many good reasons to cover an aquarium - like jumping fish, labyrinth fish that need warm and humid air above water, excessive evaporation and others. But those reasons do not apply to every aquarium. For example, the evaporation can be even a benefit, if you have air too dry in your place.
I keep most aquariums open. Aquariums in the very popular Takashi Amano style are also almost always open. In my opinion it looks better and it is very nice to be able to see the aquarium from the top.
And the last note: a healthy aquarium should not smell awful.
In my years of experience, I find it better to keep the roof closed. This way the water evaporates slower, and it keeps the water clean. But nowadays I see more and more people leave it open with just LED lights hanging on top. I have never tried, but their tanks seem in good condition too. So maybe there are other ways to maintain the tank?
And yes to a lot of the answers here, a healthy tank shouldn't smell awful.
I have a canopy on my tank because I like to keep the cat out and the fish in. But apparently my Western blacknose dace enjoys headaches. He continually jumps and bangs his head on the canopy. Several times I have seen the dace and my central stoneroller minnow with the skin peeled on the top of their heads. The spotfin shiners seem to be a bit smarter. Below are my jumpers.