Evaluating the various lake dye brands by their cost-per-gallon is very misleading.
Besides the two EPA registered lake dyes (AQUASHADE and ADMIRAL), non of the other brands may legally make any algaecidal or herbicidal claims. Doing so would require them to register their products with the EPA as "pesticides".
Lake dyes, whether a liquid or toss-pack, contain vastly different amounts and blends of pigment. The non-registered lake dyes make only one claim ~ they'll "turn your pond blue". "How blue per-gallon, and at what total cost?" is the real question.
Interestingly, all of the non-registered lake dyes use the same recommended dosage-rate of 1 gallon per 4 acre-feet, even though their dye contents are considerably different (usually much less concentrated) that the EPA registered products.
Also, AQUASHADE and ADMIRAL contain acid-blue AND acid-yellow pigments; the later of which helps to absorb specific wavelengths of sunlight.
Most (virtually all) of the cheaper lake dyes contain only acid-blue, since it is a much cheaper component that acid-yellow.
Also, the cheaper lake dyes usually contain less, if not significantly less, acid-blue pigment than their registered counterparts.
Basically, it is important to understand that two seemingly comparable lake dyes may both be packaged in 1-gal. jugs and have the same "recommended dosage". However, it usually takes more of the cheaper version to accomplish the relative degree of coloration that is offered by the registered versions. The cost-effectiveness of a lake dye is usually better for the registered versions, even though they're more expensive.
