That... depends on your budget, really, and what/how much you expect from it, like wifi or bluetooth options, how much control you want over sunrise and sunset options, etc., as well as whether you're looking to replace your lights, or just supplement them.
Depending on how old your current lights are, they may need complete replacement: when running at 100% for 8 hours a day, LEDs are really only good for a few years before their spectrum starts to shift, and even premium ones can start to show degradation after 3 - 5 years. They don’t just dim over time - phosphor degradation leads to spectrum drift, which can cause unwanted algae growth or impact coral coloration. Heat management plays a role too, since higher temperatures speed up wear on both the diodes and the power drivers. Even though manufacturers claim long lifespans, real-world conditions in a reef tank tend to shorten that considerably. Generally speaking, don't buy used LED's that are more than 2 years old unless you've got the skill and inclination to replace the diodes yourself.
As a supporting member of ARC, you're able to "rent" a PAR meter from the club, which might be worth doing both before and after buying a new light to understand your levels, and what you need. A lot depends on what you want to keep in your tank, and where. Keep in mind, you can scorch your existing corals, etc., by adding too much light, especially too fast. Heck, I just managed to nuke a
green water culture with a 60w light too close to it: in that little tank, you don't really need a lot, but the less power you drive through it, the longer it will last.
With high-end, name-brand stuff, you can expect to pay about $4 - $7 per watt. You can spend less, for sure, but the quality of what you get may be variable. Get more total wattage than you
need, if you can, that way you can dial it down.
For my planned refugium of about the same size as your 13.5, I picked up the
Noo-Psyche K7 mini for ~$84 on sale from Aquarium Specialty - though it seems to be back up to full price there for $139 now. I figure 20 watts or so is probably about enough for a ~14 gallon tank, so as this is a 60w light, I'll start it off at about 30% on each of its 3 channels and see how it does. The app isn't the
absolute worst, but it's not the best, either, and there's not as much granularity in setting times for lighting levels as I'd like.
Still, that's for my "display refugium", and not my main display tank itself. If you've the budget, you might want to look into a
Kessil A360 Tuna Blue, as it seems to be
the light to have
, with a solid app that isn't janky, amazing color rendition, and a wealth of options. It is, however, regularly $449. There are plenty of options in between. If your current lights are still okay and you just want a supplemental boost, you can look into a light bar like the
Orphek OR4 iCon, which would give you another 30w of whatever color range you feel like you're missing, or something similar.