As others have already said...
That clownfish is too large for that tank.
Live Aquaria and other reputable sites on fish care recommend a tank of at least 30 gallons:
https://www.liveaquaria.com/product/8236/?pcatid=8236
@Jay Hemdal who is an expert on fish care recommends a calculation of (Length of tank + width of tank) / (maximum size of fish x 0.8). If resulting number is less than 8, it isn't recommended.
He has an article on Estimating the Need for Swimming Space for Aquarium Fishes here:
https://www.reef2reef.com/threads/estimating-the-need-for-swimming-space-for-aquarium-fishes.816797/
Using Jay's ratio and assuming your pico tank is roughly 9" by 9" (it's probably a little smaller than that...maybe 8.8" x 8.8", but we'll round up just to give you a little more wiggle room) and using max size for the maroon at 6", the result for this equation is (9" + 9") / (6" x 0.8) = 18 / 4.8 = 3.75
Jay's minimum threshold is 8 or greater (he recommends 10 or greater), and your calculation comes out to 3.75.
This fish is too big for this tank, and it will not be ok to keep it there.