Freshwater aquarium Nitrate and Nitrite level are HIGH?

I have had this aquarium for about 3 months immediately. i change 75% of the water once to twice a month, and the filter once a month. each time i alteration the water i stir up the gravel as i get rid of the water, later i use a net to get most of the remaining debris out of the hose before adding clean hose (which i use conditioner for). i have about 20 gallons of water (in a 50 gallon tank) near two small slider turtles and a beta fish (they get along fine). i have a 20 gallon under hose (not gravel) filter (tetra whisper) and a heating element (tube type, tetra also). I have gravel, four plastic patterned plants, and a basking dock. up until the last water tweaking (a few days ago) all water tests be always fine. now my nitrate and nitrite levels are hazardously high, but the ammonia level is normal, say no stress. i recently added the thermometer to the aquarium. so could it be temp related? or what?
I'd make certain the bed of your tank is deep - if you can supply some non-silicate sand to the bottom, that can help. Also reduce how much you're feeding and devolution out the water... but when you change out water, try not to use thump water unless it's been sitting out for at least 48 hours. The chlorine within tap water will kill the microbes needed to break down animal wastes.

It's your upkeep (tank cleaning) that's the problem.
You should be changing 25-50% every week and Cleaning the filter sponges/wool every water change never replace unless they are falling apart and later only half at a time
Your tank is going through a partial cycle every marine change due to the massive amount you are changing
You want to be changing 20-25% of the water weekly. Strange with turtles contained by the tank you have no ammonia, but you have microbes or you wouldn't have nitrites.

Logically the problem is lack of surface agitation preventing gas exchange and locking the cycle. With a Betta in the reservoir that the turtles will one day eat, the solution would be an air stone which would shrink nitrates straight away and have a knock on effect with nitrites soon after.
Perhaps you are still in the middle of your cycle. First the ammonia goes up after comes down then the nitrite then finally nitrate. I would do another water redeploy to get the levels down and keep a close eye on it.
Answers:    High nitrate means you are not changing satisfactory water, no other options.

If you have turtles I propose you probably need to do 50% water changes every week.

Nitrite indicates a problem beside the filtering. As the ammonia is zero and you have nitrate that indicates you do own a partially working cycle. Just look after the filter and see if that settles down. Extra aeration may help as the cycle microbes need oxygen to do their thing. The extra marine changes that you need to control the nitrate will also help the nitrite plane temporarily untill the filter settles down.

The turtles are likely to eat the betta at some point as they get bigger.

Have you been doing the w/c's since you get the tank?

Unfortunately I don't think your tank have time to cycle.

While your tank first starts up, you don't wanna do large water change for the first almost month and a half, and on top of that, you want to check to variety sure your tank is cycling.

You'll first see the ammonia go up, after it go down as the nitrite goes up, then while that go down your nitrate will go up.

Also, when doing water changes, you don't want to stir up the gravel, that a moment ago makes everything that's trapped in the gravel come into the water more and break down faster.

You stipulation to get a gravel vacuum and clean the water that channel.
That just takes the water and cast-offs out without it completely getting out into the water.

Also, give the container a little more time to get set. Feed the turtles and the betta only little bits of food for the subsequent couple weeks.

I agree next to marcus on this one, need more Oxygen to get into the water. Also your keeping turtles so they are not the cleanest of eaters and the betta is far from a virtuous cleaner himself so you will want to do more vacuum/water changes. Your cycle is about halfway thru and you any over feeding or not vacuuming enough is the cause of the glorious reading's. Both of these can be the case and the ammonia wouldn't go up. That's my guess anyhow, hard to make clear to without your feeding info and amounts. Good Luck : )
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