Your problems are basic! surprised nobody responded. First I would not throw iron or other products on trees unless you know specifically what is going on. Brown tips on leaves is quite common, especially with avocado, it is usually moisture issues to the tree. Looks like you have weevils eating your leaves, that is also unfortunately life in Florida now, you cannot stop it easily but it will not kill the trees. I cannot see the photo's very easily so that is all I can say which is probably to late anyway for this is an old thread. Just keep it simple. keep weeds 3 ft out on all sides from trunk which is a 6 ft span total. Then mulch 4-5 inches deep is adequate, and keep them watered. Fertilize with an 8-4-8 or similar with micro's and you can get that one at home depot, it is very common, decent quality, and good for about anything and will even work for bananas in a pinch if that's all you have and want to purchase.