Hi, quindici, and welcome! It took me over a year to figure out my most meager pic skills, so if you need help, someone ELSE will be along. In the mean time, please give a more detailed description. Lots of people confuse some Euphorbia with cacti, do you have a name for what you have? No cacti are native to Florida, you just get too much rain (I lived in Orlando for a while, and though I don't know Marco Island, I'm sure it gets more humidity than any cactus
will want). Are they columnar, clumping, have leaf-like things near the top 1/3 (if so, they're probably Euphorbs), how long have you had them and when did the problem start? Is the yellow all over, just at the top or bottom, firm and dry or mushy to gentle pressure? It's winter, even in Florida, and at a first guess you are overwatering them. Cacti rest in winter, so fertilizer would not be a good thing. What kind of soil/pots/drainage/sunlight do they have? I grow mine in terracotta with a fast draining soil. In the summer they get watered only when they are dry. In the winter they don't get watered. I watered them for the last time before Thanksgiving and won't even think about doing it again until after Valentines' Day. They always get as much sun as I can give them. Don't be afraid of the Florida sun, these plants
will love it. But adjust them over a few weeks if they've been in the dark. They want the drier, cooler (but not shadier) rest in order to flower
next year. Good luck, and I'm looking forward to hearing from you!