Need help w my red ear slider tank!!!!?

I just upgraded my res tank to a 40 gallon from a 29 gallon ( i had the 29 for a week and learned that it was not large enough) He is 12 years old and big. I got a new 40 gall filter that is cartridge and i just set this tank up yesterday. today the tank is already dirty..kinda has a brownish tint to the water and it hasn't even been a day. the reason i got the bigger tank was because(so i was told) that it would cut down on the amount of s in the water. I have no gravel in it just some large river rocks on one side. What am i doing wrong... I just got this turtle from my nephew who hasn't taken care of it so i want to make sure everything is right. I did my research first but i cant seem to keep this water clean for more than a day. I only have the tank filled not even half way up. Any advice would be great!!!!!