I planted one in 1998 and it usually produces huge crops, without benefit of fertilizer or any special care. About every 5 years we get almost nothing and I panic thinking I'm about to lose it. Then the next year it comes back strong. Your tree is probably just resting.

Hi I have a Ruby Red in a pot. I got it for my dd because she adores grapefruit. It is very sweet. No need to add sugar even. It fruits really well for being potted. In Phx we had a large mature Oro Blanco grapefruit in the ground that put out way more fruit than we could ever use lol.

As with all citrus, environment, watering schedule, fertilizer, pests etc can all play a role in fruit development.

It sounds like you have inherited a nice tree. I think you should fertilize the tree. Make sure you are also giving it deep watering a few times a week and water before you fertilize. Mature citrus are usually fertilized 3 times a year. If you fertilize Valentines Day, Labor Day, and Memorial Day are a good way to remember lol.

Also, one other thing to check for is: Look at your graft line. It should be down towards the bottom of the tree. Make sure there is no new thorny growth underneath that graft line. If there is prune it off. Sometimes these suckers from the rootstock will take over and make the original tree less vigorous and can sometimes affect fruit quality.

Here is a good link for info on citrus culture

Here is a link that might be useful: Citrus Culture

Most Florida soils are deficient in magnesium, and citrus need it. If the fertilizer you are using doesn't have enough, you can use epsom salts. In my yard, I use what the company calls SoPoMag, which is 20% sulfur, 20% potassium and 20% magnesium. It is an artificial version of a naturally occurring mineral. I can't remember the name right now.