I believe you are cleaning your filters to often among other things. You certainly have enough filtration on ths tank. Here is what I would suggest and things to look for. I have wriiten about this next subject many times on the forum. The intake strainers for your filter's should be at the gravel level. If they are half way down the tank, I would suggest getting extensions for them and get them to the gravel level. In your first post you state that the nitrate looks good. It should on a new tank. Nitrates take awhile to accumulate. What do your nitrites and ammonia levels look like? These are going to be the tell tale signs in a new tank. When you change the filter cartridges are they dark brown or black in color? If so they are normal, this is fish crap on the cartridges. Or are they reddish in color and caked up on the outside? Does the cartridge look like is it covered in snot, sorry it is the best way to discribe this. If the answer to the last two questions are yes it is reddish and caked up and it looks like snot.... Your are feeding not only your fish but the filters as well. This is a classic indictaion that uneaten food is getting into the filter and it is the cause of cloudy water. That is why the intake need to be at the gravel level. Hopefully the food will be eaten before it gets to the bottom. I would change one filter a month and then the other filter the next month. NEVER clean them at the same time. When you feed your fish are you feeding flakes or pellets? If you are feeding flakes, do you crush them up before putting them in the tank? If you do - don't. Put them in whole and let them float. Same goes with pellets. Are the pellets sinking or floating? I don't use any sinking pellets at all. Crushed food sinks quickly and goes, yep right into the fliter. Do you feed frozen food? If you do, do you thaw it out and "lightly" rinse it in a fine mesh net first? If not I would recommend doing so. Never take frozen food and drop it into a tank. Always strain and rinse it first. In closing, I hope this helps. I have two over the back filters on two tanks and I have not cleaned them since Christmas. When I do clean them it is on the off week for water changes. The water flow out of the filter is good and with doing 50% water changes every two weeks, the filter media's life is extended. The tanks are crystal clear. Good luck and I hope someone here hits the nail on the head for you.
Lonny