Whether you have an aquaponics system where you have fish as a source of protein or you just caught some fish while out fishing, here are the basics of cleaning a fish before cooking.
Depending on the fish, it could have scales or a slippery outside layer. Using something like the back of a knife, in a firm steady motion, scrape from tail towards head to remove scales. When done, wash away loose scales. If your fish has a slimy natural protective film, under running water rub until no longer slippery.
With a very sharp knife, slice along underbelly of fish from anal opening near the tail, towards the head, stopping at base of gills. It should be a shallow cut, as to not puncture any of the insides (ie. intestines).
Where you just cut, spread open enough to be able to pull out the insides, innards, entails or whatever you want to call it.
Some fish have a dark colouring inside after removing everything. If yours does, use a knife and scrape this away. Some fish also have a kidney near the spine. This too can be scraped out and removed.
Cut out the anal opening.
Give the inside a good rinsing.
You can either leave the head on and cook it, or remove it. To remove, cut just behind the gills, pushing straight down cutting the head clean off.
You can now cook the fish like this, or you can fillet it first.
Discard of the fish remains in a proper manner, to not attract wild animals.
Be careful when handling the fish as some have sharp parts (ie. fins, teeth etc)
Always cut away from you, and never have your hand in the path of cutting.
Use gloves to have a better grip and help protect against anything sharp.