How to choose a light panel size for your vivarium

Here's some things to think about when choosing a light size for your vivarium.

In general you want to use/cover as much of the top of the tank as you can with the light panel so that the light is spread out over a wider area.   Because the various niches in the glass box hobbies all have different approaches to vents and misters, front or top opening, glass tops or screen tops, a lot of variables are in play.

So what you need to imagine is the maximum usable space you have available to sit the light on the top of the tank where it's not covering a vent (even though this can be done) and its not in the way of misters.  You would then want to measure that space from left to right, and then front to back to get the dimensions (width x depth) for the light best suited to your situation.

For instance, most 18x18x24 vivs can accommodate the 16x13 light, because the 13" depth allows for either front or back placement of both misters or a vent.  But some people do have situations where they need the 16x10 panel because of all the stuff on top of the tank.

I generally choose a width (left to right measurement) that allows for an inch on either side of the viv.  It's not necessary to have a panel that exactly spans the top of the vivarium, but for some people, that's how they like it.  Based on this approach, the recommended size for 18x18 top vivs is the 16x13 panel.

Once you have the general size figured out (the footprint for the panel), then we need to consider the height of your tank, which is critical to know because it greatly impacts which light you choose.

Almost all the time the LED strip only lights work fine up to 24" in height.  Some people will quibble over whether that is too much light , not enough light, but for my purposes I've found it to be a very good solution (I've run strip only lights over a 28" high viv with what I considered decent results, but everything at the bottom was a low light plant). 

I'm also working from the perspective that there can be too much light for some species, and most of my tanks house dart frogs.  It's important to remember the natural light conditions in the habitat where the species you keep is found in the wild.  Some consideration should be made for this and in my experience most dart frog keepers do not keep this in mind when they go for maximum light and plant growth over a more natural light cycle that would benefit the animals more than the plants.  These lights were designed to be a balanced solution for both plants and animals.

Once we get up to the 36" height, that's where LED spotlights of some sort are needed to get enough light down to the bottom third of the tank.  I currently have two options for how that can be done:

1. Either using what looks like a traditional LED spotlight (MR16 base, 15w bulbs)
2. Or, large diodes that sit on a heat sink and fan unit.  Those come in 10, 20, 30 and 50 watt increments, so you have a lot more options there than just 15w.

While the large diodes might seem like an obvious "no brainer" choice, here's some reasons why you might want to consider the MR16 (traditional style) spotlight approach:

1. You don't like the sound of a fan running that you have to accept when you go with the large diode option.
2. The large diodes, with the heat sink and fan are a more complex solution with moving parts that are destined to fail at some point.
3. The MR16 spotlight, in my experience can be put over the top of even something as tall as 48" if you do not have a high light requirement for the bottom of the tank.
4. The MR16 spotlight produces less heat than the large diodes.
5. If an MR16 bulb goes bad, it's easily replaced.  Replacing a large diode is much more complicated (soldering required).

A positive side to the large diodes is that they can very easily be placed just about anywhere on the panel you'd like them to go, so there is more flexibility there over the MR16 spotlights. The large diodes also allow for a very low profile top, only 1 inch in height, which is more efficient and attractive than the "cans" you see in the photo at the top which house the MR16 spotlights.

This post could get very long if I went into every nuance, so I'll try to keep it this short/long, and hopefully you now have a better feel for what is the "right size" for your vivarium.