You should find all the info with the link that is supplied. I have three of these that are not anywhere near the age of yours (8years). The only fertilizer they are given is a weak, very weak drink of 20-20-20 orchid fertilizer and that is only once a month. What I do give them is a good size chunk of banana peel as they love potassium. Hop this helps a bit.

Here is a link that might be useful: Madagascar Tropicals