Regardless of the tank size, you will need to consider several factors concerning lids. The first question should be; do you intend to keep coral or large fish? If you intend to keep coral then you need to determine an appropriate lid thickness. It is not uncommon to see aquariums, which feature coral, running no lids at all. This is because glass reduces light penetration. The thicker the glass the more light lost. There are claims that 6mm glass lids will reduce light penetration by up to 25 Percent, With the added growth of algae and condensation on the lids, the success of keeping coral is further reduced.
Having no lids at all creates a number of issues. Without question it limits the type of marine creatures able to be kept. Some fish (any that jump), along with eels are amongst those that cannot be kept without lids on the tank. Water loss is also an issue, Either by splashing or evaporation, water loss is much greater without lids. This is especially so for heated tanks. It also keeps the heaters working overtime in maintaining temperature not a good thing for the power bill, or the heaters.
The next thing to consider is corrosion in the room. The increase in humidity can lead to extensive damage of metal objects, especially metal stands. Metal stands require high levels of maintenance that only increases with running a tank with no lids.
Although there are benefits of extra light for plants and coral, running no lids can be a definite disadvantage as indicated above. However, there might not be much choice when keeping certain types of corals. Before investing in expensive coral check the light requirements and the suitability of the coral to your tank. This can save you unnecessary cost and stress.
If you intend keeping fish, especially large ones, then lids are a must. Although light is not as much of an issue, strength of the lids will be. Large fish require strong lids. If you intend keeping fish longer than 250m, then I recommend you use 6mm lids. The larger the swept area also, the thicker the glass is required to be. Just remember, you will need some smaller, easy to access lids for feeding. Do not fall into the trap of making these lids light duty. Fish can be powerful. They might not intend hitting the lids but if they do the glass needs to be strong. If you are not concerned with light, make your lids as heavy duty as possible.
Making your tank escape-proof is a good idea. If you wish to keep any sort of eel, nothing is more important than sealing the tank. All lids need a support or something to sit on. If you are designing the tank then use the whole inner perimeter for lid supports. This strengthens the tank whilst providing a sealing edge around it. The only gap left then is between lids and this should be made minimal. If any lids need to be modified for plumbing or alike, cut glass to minimum clearance. If there are any noticeable gaps then they can be sealed using some screen mesh or filter media. There is nothing more distressing than finding your pet on the floor. So prevention is a must, as a cure is unlikely…