I thought as this was a DIY section i might post some expired patents. I often browse patents to get some ideas when I want/need to build something. First a word of caution..... Patents are not checked to see if they work!! Hence why you can find all kinds of contraptions that will never work! There is nothing wrong with looking at or building/copying something from ANY patent. What you cant do is copy a live patent and make it a product. Patents expire for loads of reasons, the main one being it can cost alot each year to keep it active, if the inventor isnt making much money from it then often they stop paying the fees. Expired patents you can do what you like with, except try and copy it and patent the idea... Anyway I will post some as I come across any of interest to fish keepers, you find all kinds of filtration systems that never got to market, one system is what i based one of my sump filters off. I didnt copy the entire idea, i just took one element from it and added it to my system. It works really well. Patents are also where you find what aquarium companies stick in medication . So if anyone finds any of interest post them here.