HI Fish Geek,
To keep things simple the drop checker is small vessel placed inside the tank which holds perhaps 1/2 teaspoon of water. A reagent is added to this water which measures pH. As the pH of the vessel’s water falls (becomes more acidic) the color of this water changes from blue (more alkaline), to green, to yellow (very acidic). Ideally, when the color is green this indicates that the optimal useful amount of CO2 is dissolved in this sample water. You can see typical drop checkers here:
http://www.aquaessentials.co.uk/index.p ... op+checker
As you can see, this funnel shape facilitates the ability to invert the vessel and to place it inside the tank while trapping a small amount of air. With the funnel opening facing down this trapped air isolates the liquid in the vessel from the tank water, however, as you inject and dissolve CO2 in the tank water it then also finds its way into this bubble and then dissolves into the vessels water. At some point equilibrium is reached between the CO2 concentration in the tank, the concentration in the bubble and the concentration in the vessel’s water. The reagent changes color as the carbonic acid content of the vessel’s water changes. I guess that since you mentioned pH/kH tables you are aware of the relationship so it should be no surprise that the water in the vessel should be composed of DI/RO (De-ionized or Reverse Osmosis) water adjusted to a kH of 4 to avoid the uncertainties caused by other buffers being dissolved in it. kH 4 water is now being sold but you can make your own. Essentially, with this water the pH/kH/Co2 table become relevant again and at around 30ppm dissolved CO2, the reagent turns green due to a pH of 6.6. My numbers may be off but this is the general principal. Origin of the term "drop checker"? I can't imagine it's anything more exotic than the fact of adding carbonic acid to the reagent water causes the pH to drop.
You should subscribe to the Barr Report as it’s money well spent ($13 per year which is peanuts) The subscription fee will give you access to various articles which address these very same questions. You can browse the site in any case for threads. The first step is to read, read, read and then when you’re done read some more.
Not everyone uses EI and those that do often adjust it’s use for their particular situation. You should not think of EI as some kind of dogma or requirement. It is a method of ensuring that there is always sufficient concentration of nutrients to provide growth, With the advent of more powerful lighting we have seen that this light is the motor which drives plant metabolism. To use the motoring analogy, imagine a car on the motorway moving at 100mph. The plant is the car and the light is the motor. What would happen if most of the engine oil were consumed? What would happen if the petrol level dipped to the “E� Think of the effects of the loss of transmission fluid, brake fluid, tyre tread on the performance of the vehicle. None of these losses can result in better performance. You might try putting in just enough petrol or oil to get you to the next station but what happens if you got the levels wrong? Wouldn’t it be better to always have a full tank, full oil, full tyre tread etc? Yes, it might cost a bit more and could be considered “wasteful†to replace the tyres before minimum tread but consider the consequences of miscalculations at 100mph. The same consequences apply in your tank. When you deplete a nutrient that the plant required for top performance. The equivalent of a motorway crack-up in your tank is poor plant health accompanied by virulent and persistent algal blooms. Most EI devotees find that “wasting†a few grams of powder is preferable to the consequences.
CO2 – As plants consume light and nutrients they “exhale†Oxygen. A the water becomes saturate with O2 no more O2 can be dissolved into solution and it comes out of solution as in gaseous form. This is called pearling which has become some sort of Holy Grail or Aquarist Nirvana. You should think more about plant growth rate. For example if the water temperature is very high such as found in a typical Discus tank the water has less ability to hold O2. Such a tank may pearl more readily than another tank kept at a lower temperature, yet the plant growth in the tank with the lower temperature may actually have better growth so pearling should not have the priority that it seems to have.
Fertilizers – I’m not at all familiar with trace mix you mentioned but really there is not a tremendous amount of difference in the various mixes. The important thing is that you dose the proper concentrations and at the proper frequency. You didn’t mention whether you dose N, P or K but if not you will need to. These are top priority nutrients and are referred to as “Macro†nutrient since they are typically dosed in relatively large quantities the chelated traces are to plants what vitamins are to us.
The phosphate discussion you mention became complicated because we are all still trying to interpret what we can physically see in our tanks within the context of the things that we can’t see. This is another reason EI is so useful. In fact the original poster of that thread was concerned with excess just as you mentioned that you were concerned with wastage. In fact EI was not the result of brainstorming. It was developed by someone who discovered, through his profession of working with water weeds and algae that excess nutrients doesn’t cause harm but that nutrient deficiency always causes harm. His conclusion was therefore to dose in a manner that never results in nutrient deficiency. If you follow this principle an if you never worry about wasting or exceeding you have a better chance of avoiding trouble.
Test kits – In my opinion – Generally wasteful and irrelevant. I was once enslaved by the allure of test kits. I would pretend to be a scientist. I even completed the fantasy by wearing a white lab coat. I thought I would have an accurate picture of my tank parameters and that my plants would bask in the glory of nutrient bliss. It was a disaster. I was like a refugee as I mindlessly immigrated from one lame test kit to another, each giving more pathetic results that the previous one. I used fantasy concentration levels to calculate fantasy ratios and it was as if I were at the horse races. Some days, if my test kit was feeling good, I would score big. On other days, if the test kit hadn’t slept well, I would be the (no-so-proud) owner of an algae farm. Again, EI comes to the rescue in that you no longer need to test for nutrient concentration because you know that the concentration is sufficient. Who cares what the actual values are? I admit though that I always like to know pH, hardness, and more importantly, TDS (Total Dissolved Solids). These are parameters whose absolute values I feel are relevant to both plants and fish. I bought the Hana Pocket Combo meter which measure pH, temperature and TDS. It is supposedly accurate to within 0.01 pH (yeah, sure). As long s it generally agrees with my drop checker I’ll believe it. I find it very handy. A kH test kit isn’t too bad but I constantly attempt calibrations. For example, if I buy a 4 dkH water I make sure that the kit returns that value when I test. If I have a DO water sample the test must return a zero reading. If you want to talk about wasteful just check the prices on some of these kits. I’d rather use that money to buy more plants. Remember the drop checker also uses a reagent from a CO2 test kit. This is just another pH test kit though, in actuality masquerading as something more amazing.
Apologies for rambling. It also just dawned on me that if you don't drive, the automobile analogy may have been a complete waste of time. In any case let us know if we can clarify anything else.
Cheers,