A centrifuge is a very useful piece of equipment for isolating aquatic organisms. Centrifuging a sample of water containing micro organisms will drive them to the bottom of the container, where they become more concentrated and easier to manipulate. A high speed laboratory model costing several hundreds of dollars isn't necessary for the purpose, something much simpler will do the job quite adequately. At the most basic, you can just tie a piece of cord SECURELY to the container and whirl it around your head. You can easily generate a centrifugal force of 10 G with this method ( but do it outside in an open area, where nothing will get damaged if it comes loose ! )
A safer and more convenient solution is a motor driven apparatus, such as the one I built using parts from an old video recorder. It cost me only a couple of dollars for the plastic bowl housing the rotor head and some PVC pipe fittings, all the other parts were obtained from my “junk collection” of defunct VCRs and other “dead” electronic gadgets. If you don't have a dead VCR lying around, just ask a few friends and you are bound to find someone who will give you one. The parts required are the video recorder spinning head mechanism, plus the video cassette loading motor with its drive pulleys. Some newer VCR models may not have belt drives, a good alternative source for these components is an old inkjet printer, but these usually have stepper motors which require a special driver circuit to power them. If you can find an ordinary DC motor which will fit the pulleys it is much simpler than trying to get the stepper motor working. You will also need a 12 Volt DC plug pack. It should be capable of supplying at least 400 milliamps, otherwise it will probably burn out before long. If you need to, you can buy a new one for about $ 5, but these are very common items found in garage sales for only one or two dollars.