Top-fill or bottom-fill?
There some finesse tricks to each tank. I can fill a Kayfun and get no leaking, but it took a while and finally reading the "trick" that did it for me. The issue is they rely on vacuum to keep the liquid from coming out the air intake. For the kayfun, I botom-fill and suck on the driptip before and as I turn it upright.
Ultimately though, the mess and wasted juice of RTAs led me to bottm feed drippers. I've been much happier with my vape since.