theotheragentm,
Bioload is the key issue in healthy plants with little fert addition. If you have a high bioload your nitrogen levels, and most likely traces are adequate to grow most plants in most conditions. CO2 levels will still be a potential problem, especially outside like in your goldfish barrel (due to high light from the sun), but everythings possible.
Your current 55 is another case, however. You mention that it takes 2 weeks or so to see nitrAtes. This could be problematic in the future if you add pressurized CO2 or increase the lighting. You would then probably need to dose additional nitrogen.
Here's my fert basics guide written on the fly:
Your plants (just like us) require nutrients to not only survive but thrive and fight off disease. Many are found naturally in your water supply and from fish waste. These include macro nutrients (those that are needed in large supply) such as nitrogen, carbon, potassium, magnesium, and calcium. In many areas many of these nutrients are found in abundance in the water and food that is fed. Think of these like your grains and fruits/veg's/meat. They are needed in large amounts to keep the plant healthy. A deficiency in these can cause the plants to become wounded and leak nutrients into the water that allow algae to become apparent. Our goal is to have these nutrients in ample supply so we never have weakened plants, and thus, they outcompete the algae for food.
Just like our sweet-tooth side, plants also need their "fats and sweets". These are needed sparingly, but are needed nonetheless. People die without fat. Too much is a bad thing, but too little is much worse. With plants trace elements such as Iron, Manganese, Boron, Chlorine, Sulfur, and a host of others are needed for healthy plant structure. Most of these are found in adequate supply in your water source and through the food and fish waste produced. In low light setups and most water supplies, supplimental trace ferts are not required. In higher light setups, however, the small amounts of traces in the water/food are not enough to prevent deficiency and so you need to add these back into the water during the time between water changes.
So to quickly recap, you have macro nutrients that are needed in large amounts, and you have micro nutrients that are needed in small amounts, but JUST as important to the overall health of the plant. If these are kept in adequate amounts, your plants will outcompete the algae, and you should not see much algae. This is no simple feat however. All it takes is one minor deficiency and you can have an outbreak of algae on your hands. The plants grow slower, or leak nutrients into the water, and suddenly you have given the algae everything they need to quickly take over the tank. This is just like our own health. We are constantly being bombarded by disease and infection and normally our bodies are able to fight these off. But eat poorly, stress out, and get too little sleep, and you will get sick.
The EI method attempts to provide excess of all the nutrients so you never have this deficiency problem. By adding in more then is needed, there shouldn't be a chance for your plants to become weakened. Every water/food source is different, and you need to compensate for individual source deficiencies (and its often hard to find out what your water doesn't have, a report from your local water supply is a good start). Hard water is generally a good sign that your macro's are in pretty good numbers, but its not a guarantee.
2 macro's that are normally NOT in proper supply once the lights get turned up a bit are potassium (K) and carbon (CO2). Potassium is that one macro fert that causes so many problems because its normally not dosed when other ferts are not dosed (and not as heavily found in water). CO2 is self-explainatory. As the lights turn up, that 1-3ppm of atmospheric CO2 (and sometimes even with Excel) just doesn't cut it, and it will become your limiting nutrient.
And that's where it gets so complicated. Potassium might be your limiting reagent, but once you dose it, the next in line becomes the problem (say iron). You now dose iron and phosphate is in short supply. Compounding all of these problems is our test kits are not very accurate and might be measuring nutrients that are not in the proper form for the plants to use. Just like we can't use the oxygen in water (we drown), plants might not be able to use some of the nutrients that your test detects. So even though that test says you have 1ppm of phosphate, your plant might not be able to use it! By dosing all the ferts in excess (within reason) you make sure the plant always has access to the nutrient it needs.
HTH
Keep asking specific questions and we'll keep answering.
