For most aquariums with 3+ WPG the plants will burn through the macro nutrients so fast that you have to supplement. There are a number of variables like the size of the aquarium and how much light is actually making it into the aquarium, that affect whether or not 3 WPF is actually high light or not. Occationally there's a high light aquarium that has a high enough bioload and/or nutrients in the tap that it isn't necessary, but you don't hear about those too often. Generally you should only need a good Trace Fert which will have plenty of iron, Nitrate, Phosphate, and Potassium. Depending on the water supply some also need to dose Calcium and/or Magnesium (low GH and/or out of balance). It's definately better to have the nutrients on hand and not need them, then find that you need them and end up with an algae mess before you can get ahold of the necessary suppliments.
You're Nitrates could be climbing for one of two reasons, either a high bioload or a shortage of another nutrient which is preventing the Nitrate from getting used. If a shortage is causing the Nitrates to climb, when the shortage is resolved you could find that you actually need to dose Nitrate to keep it from bottoming out. The 10:1 Nitrates to Phosphates ratio is a good starting place, but can be adjusted to fit your aquarium's needs. I find that I'm probably dosing closer to 5:1 in order to minimize GSA. For Traces you want to dose enough to achieve 0.1-0.2ppm iron and for Potassium you want to dose 10-20ppm. Again these can be adjusted based on the needs of your aqarium. It's all about dosing at a good starting level, and then adjusting as needed based on the needs of your aquarium, which will vary depending on the types of plants you're growing at a given time and how recently you've trimmed.
Personally I'd recommend upping your waterchanges to weekly now, instead of later. They wouldn't have to be as large (maybe 25%) as required for EI (50%), but it's a good habit to be in.