Rubber maid tub throwing chemistry off?

[Reef keeper]

Okay if this is the wrong section for this then please move. I have a 400 gallon with a Rubbermaid tub as a sump, theirs 2 eels and 4 clownfish, lawnmower blenny and 4 monos.
This is my issue theirs barely any bio load what so ever, I have a skimmer rated up to 750 gallons but my nitrates are astronomical. They are 100 on the salifert kit. My phosphate is .16 if I don’t change the GFO every couple days it sky rockets to .4-.5+

mag is 1280
Calcium 430
Alk 7.8

My RO goes through 9 stages 3 sediment, 3 carbon and 3 DI and tds is 0.

I’ve been doing 50 gallon water changes 2 times a week and haven’t even touched the nitrates. Their are nitrites that keep coming up to. I feed once a day Flake for the fish, 3 shrimp feed directly to the eels. The tank has been setup for 9 months now.
The only thing I can think is the Rubbermaid is leaking something in. It was brand new when I got it.

looking to see if anyone else is having issues with these tubs

 

[mehaffydr]

I can't imagine that a Rubbermaid tub could possibly put Nitrates in your water but lets see what others say. I do use a Rubbermaid 150 gallon for my sump and have low Nitrates to the point I have to dose.

 

[Randy Holmes-Farley]

What tub exactly?

It is highly unlikely that any plastic is pushing nitrate super high.

"I’ve been doing 50 gallon water changes 2 times a week and haven’t even touched the nitrates. Their are nitrites that keep coming up to."

How much nitrite?

A little nitrite (say, 1 ppm) can read as 100 ppm nitrate, depending on the kit.

The tank may just not be able to deal with that much food every day.