Others have posted cheap coolant systems, this system is for small machines, milling, lathes, grinders.
I made this one for use with my Seig X3 milling machine, it also uses a plastic tank and a fish tank pump and has a filter system to stop metal damaging the pump. The filter is simple three stage filter. Stage one is a corse membrane to collect large objects and the second stage is for settling heavy particles before the overflow to the final overflow tank and is returnd to the main tank where the pump is. In the final settling stage I placed a very strong magnet to collect any ferite material that may be small enough to float on the miniscus of the coolant or light enough to have passed through settling tank.
I made the complete filter system out of plastic and is removable (simply lift out of the main tank) and clean. For cleaning the magnet I DO NOT advise compressed air as this will have very fine matal particles airborne and traveling at high speed, fast enough to penertrate your skin. Just use a cloth and wipe off as much of the filings as possible. The stronger the magnet(s) the better, if you want them to stay in place when you move the filter in/out of the tank just glue a ferite plate under the final settling stage and the magnet(s) will stay where you put them even if you turn the filter system upside down.
I hope to have photos of the filter later as well as a grinding attachment for the X3 mill, which was the reason for the filter in the first place.