Here is whether you should add a lid to your wish tank or not.

 It is sometimes a good idea to use cover glasses on marine aquariums (oft-hinged glass or acrylic lids) and sometimes not. They are available in various dimensions to fit tightly on top of aquariums of different sizes.

As with a lot of aspects of the marine aquarium hobby, there’s no such all-encompassing wrong or right answer to this question. It is sufficient to say that cover glasses may be appropriate in some of the circumstances but totally inappropriate in any of the others.

For determining what’s best for your system, consider the following cover glass pros and cons:

Pros:

• Having an aquarium lid in place reduces evaporation to a great extent, which in turn reduces the frequency and size of freshwater top-offs and helps lower the humidity in the room housing the aquarium.
  
  
• Fish prone to slithering or jumping out of a tank are kept in the aquarium where they belong. Some fish, like eels, and even certain invertebrate species like octopuses, are such good escape artists that a tight-fitting custom aquarium lid is a must while keeping them. However, for a lot of fish species, there are alternatives to acrylic/glass lids that might do the same job, e.g., covers made of some kind of mesh or plastic egg crate or screening material.
  
  
• The light fixtures are better protected from corrosive salt sprays and splashes when a lid is intact. It becomes especially important if the fixture is not equipped with protective lenses to shield the bulbs.
  
  
• Salt creepers don't advance as far when a covered glass is in place, though it still builds upon the cover itself.
  
  

Cons:

• A red sea lid or any other air-tightened lid can reduce vital gas exchange at the water surface. You can compensate for this by giving an open sump and providing rigorous protein skimming.
  
  
• Salt creep and Lime deposits on an acrylic/glass cover reduce light penetration, which is detrimental to invertebrates that are photosynthetic. So, regular cleaning of the cover glass is essential to prevent this build-up which adds up to your overall maintenance cost.
  
  
• A tight-fitting cover tends to trap heat that can result in unacceptably high water temperatures, especially in warmer seasons.
  
  
• Evaporation decreases. Yep, this can become a con as well as a pro because it also means that the evaporative cooling going on is less than further contributes to heat build-up. A slow rate of evaporation can also become a con for reef keepers who drip kalkwasser for supplementing alkalinity since saturated kalkwasser is typically used for freshwater top-offs