I am starting to read this very old book that my grandpa had on tropical fish from front to back cover and the guy says that there is a way to figure out how much water surface your fish need for oxygen. Some of the measurements were for small fish like guppys they need 1.5 by 1.5 inches of water surface so every guppy you had needs that much square inches of water surface and large fish like cichlids that are around 5" needs 6 by 9 square inches of water surface to be comfortable in oxygen. Remember this is calculated without extra areation and a tank at around 76dgrees Farenheit i belive. What are your opinions on this, Do you think its a good rule. I will go get the name and author of the book soon and correct anything if i did it wrong and keep writing interesting facts i find so I will keep the post as like a diary. Feel free to write anything you want on here questions or comments.