This is what I was just fighting in my tank due to exactly these reasons. Low nitrates and phosphates generally cause dinos. Mine were undetectable, and thus dinos. The best solution for me was to increase feeding, 2x a day, and then adding Tim's Waste away daily. It takes awhile to get dinos, so it will take a month or two to beat them fully. Im almost done with them now. Best of luck!About 98% of the time,dino's kick in when nutrients stay bottomed out at 0ppm.
A lot of people wonder why I dose NeoPhos/Nitro in a newer tank,but this is because after the cycle I added a CUC and thats it. After about 5 months,nutrients are still at 0ppm. So instead of fighting dino's for months,I just dosed nutrients to prevent this from happening.
Also,can you post a picture of the tank please.