I got a Betta, two weeks ago. He is doing great in his 8 gallon tank set up as a freshwater tropical tank instead of the stupid little blue water bowl.
The tank was an established tank, 100% water change before he was added, he has the tank all to himself, but several days ago the water became rust colored, I did about a 90% water change and a day later it is rust colored again. I have no idea what is causing water discoloration.

if too much food or waste is produced, the fats and protein off the food put a tinge to the water and a very thin layer of a yellowish slime on the wall. Try feeding him less. Get him betta food (little pellets) feed him 1 or 2 a day. Nothing bigger than his mouth can eat and only feed them 1 at a time.

without knowing what is in your tank, we can only guess... but the first thing that comes to mind is driftwood. if you have a piece of driftwood in your tank, it will leach tannins into the water. if this is the case, its harmless and as time goes by it will leach less and less tannins, so you wouldnt have to change water as often. tannins can also come from peat moss, but that usualy leaves a more yellow tint, like a light tea. the other thought that comes to mind is certain free floating algae can appear brown under certain light, but thats a bit rare to run across. water changes would help fix an algae problem, along with proper filtration and avoiding overfeeding.

yes i had peat moss and overfeeding problesm both on seperate tanks and both made a nasty colored water

FYI,A bettas stomache is only as big as his eye.I agree,feed less.If you feed flakes,switch him to pellets,its better for him anyhow.

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"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

Talk about not seeing the forest for the trees. I did buy a piece of driftwood and add it to the aquarium. I will leave it in and just change the water out until it clears up, the betta likes to hang out in the hole in it, so if it is not hurting him I will let him continue to enjoy it. Thanks for the info.

adding to this ^ only feed a pellet that is small enough to fit into his mouth or it'll spit it out and be done with it

this could be food or waste that has been there for awhile, or tannins leaking from a piece of driftwood maybe if you have one

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Current Tanks:
90G,30G,10G,10G,5G,38G,20G

personaly, i like the dark water look that the tannins give to my fish tank, so i add oak bark extract(i make it myself) to my main community tank on a regular basis. it seems to make the color of my fish stand out just a little more, against the darker background. plus, every time i add it, my fish seem to calm down a lot. i cant add it all the time though, since it does block a lot of the light reaching my plants.