I've got three tanks set up - a 29G, a 25G, and a 10G, all with gravel, lots of plants, and a light fish load - a very light fish load. the 10 only has two guppies in it! Its chemical parameters are the same as the 25G tank, but it keeps getting cloudy and I have to do an 80% water change every week. The plants are healthy. I cannot figure out what is happening! The 25G has a small HOB filter, one for a 10G tank; neither the 29G or the 10G have a filter. Outside of adding a filter, I do not know what to do - and if I do add a filter, I don't know which kind to add. I'd like to make the 10 a guppy fry tank, so I'm thinking a sponge filter.