Purple tang - I know it "seems" like their isn't anything wrong with your system... but a stocking level like that is going to cause problems. Whether it be by simple water chemistry, (IE nitrates from all the waste those fish produce or simple ammonia or nitrite problems from the massive bioload) abnormal algae growth from those nutrients, increased aggression levels as these fish mature into their TRUE nature, and the increased stress from conspecifics creating an outbreak of diseases when something goes wrong (say, your UV sterilizer goes out on you).
If you want to test your theory and play the "my tank is alright" card, then try unplugging the UV sterilizer. In a properly stocked aquarium, fish should be able to maintain their immune systems and not break out into an all out ich fest. In an overstocked aquarium, you get stress, stress lowers immune systems response, and you get ich. So turn off that UV sterilizer, and if your fish are ich free in 6 weeks... by all means, call me a lier.
By the way, you should check out my stock list for my 210 gallon to get a better sense of how many fish should actually be in a tank that is so large. Let me tell you, its WAY less than 30 including gobies and such.
I want to know where you got the info that 30 fish would be fine in a 135 gallon... because they missed some key things... such as territoriality, dissolved oxygen, size of the fish, behavior of the fish, diet of the fish, ect, which all play a role in the choosing of a fish and stocking level of an aquarium. For instance, I have a 210 gallon, techniqually I could stock it with around 50 fish, but they'd all be tiny gobies. Where as I choose 3 tangs, and that eats up around 10 gobies bioloads because of the factors above.