BUGS IN OUR FLEXARIUM?

a week ago i noticed some tiny (minute)black bugs also some larger red ones (like red spider)in our flexarium which houses a 2yr old iguana,they were also on the iguana , i proceeded to strip the flexarium out cleaned everything ,bathed &showered iguana to make sure he was clear few hours later all was well until today same thing again . don't know why they are there and if they are harmful to our iguana don't understand why when we have reared him for two tears with no problem's ,can anybody help ?