Definitely recommend the group “Reptile Lighting” on Facebook- it’s run by Dr Frances Baines, who has been involved with a lot of the research into heating and lighting for reptiles.
Please let me know if any of this doesn’t make sense / is phrased confusingly, I just woke up and don’t have anyone to proofread for comprehension 😅
It sounds like you’re using coil UVB, is that correct? I would get rid of that. Coil bulbs produce a small, concentrated basking area with a high UVI. This is great for a small, diurnal, open basking species but not ideal for larger, crepuscular geckos that typically cryptic bask (like cresties and gargs). I’d replace them with a linear T8 or T5 tube, which will create a larger and gentler basking zone. You want your gecko to be able to fit their entire body into the basking zone provided by the UV and the heat element.
For low UVI-needs geckos in a short tank I’d use something like the Arcadia Shadedweller Arboreal (2.4%) bulb. It’s a slim 12 inch bulb which is perfect to create a lighting gradient but also a sufficiently large basking area. Through Exo Terra mesh, at a distance of about 4-6 inches, the UVI will be 0.7-1.1 with this specific bulb, and then down to 0.1 by the time your gecko gets 12 inches away- so the bottom of the tank will have no detectable UV, but the very top will be at the top of the range for Ferguson Zone 1 geckos. They’re unfortunately not super cheap (about $70 for the fixture and first bulb, and then $20 for replacements, in the US at least) but Arcadia’s bulbs tend to last the longest on the market so you may not need to replace bulbs until 1.5 or even 2 years rather than yearly. This is what I use for my own crested gecko in a 48x24x24 inch tank.
CHE is fine to stay, you want it aligned as closely as possible with the UV tube so that the infrared from the CHE and the UVI from the UV light overlap, creating a sunshine effect- for many reptiles, bright light is a signal to seek warmth in that area.
Since gargoyle geckos and crested geckos are both generally partial baskers, make sure especially that there’s plenty of cover such as foliage, branches, and hides under the UV light. This way they can easily choose between full exposure, partial exposure, and no exposure. It’s typical for these geckos to hide with just their tail or one leg exposed to UV to cryptically bask while they sleep.
Depending on what plants you’re trying to grow you may then need a third plant specific bulb for the full visible spectrum. I’ve generally not had much trouble with pothos, spiderplants, and tradescantia under just a UV bulb and a halogen bulb, but fussier plants might need a little extra. I like the Arcadia Jungle Dawns but they produce a fair amount of heat and are expensive. I’ve also had luck with Sunblaster brand plant lights, and Vivarium Electronics brand too. Some of these companies don’t make short plant lights, so you could get a 3 ft bulb and place it on top of both tanks, if they’re sitting next to each other.
I do as a general rule recommend halogen bulbs for daytime basking and CHEs only to raise ambient temps and for nighttime heat, but in an 18x18 tank you may have difficulty finding a bulb that will produce a large enough basking zone without overheating or needing to be dimmed excessively (halogens are best when dimmed as minimally as possible). Since cresties and gargs are fairly temperate animals it’s not the end of the world for them but I still recommend halogen heating if possible. My crestie gets halogen in the winter during the day but during the summer it’s too hot for an additional heat source
