Actually they all have a small amount of phosphates in them. Some people go out of there way to actually rinse the food off in ro water. I just phosban in a 2Fishes reactor and I have no problem. I feed the fish every day no worries.
Many seafoods of all types have phosphates added to "preserve freshness". Sometimes this is noted on the label, but for fresh seafood no such notice is provided. Dry fish foods do certainly have a lot of phosphate in them.
Organic phosphorus compounds, as well as orthophosphate, are so prevalent in biological systems that any natural food necessarily contains significant concentrations of phosphorus. Not only can organic material be taken up directly to provide carbon, nitrogen and phosphorus, it can be broken down by organisms and released as inorganic nutrients, such as orthophosphate, ammonia, nitrite and nitrate. The metabolic breakdown scheme for typical organic materials in phytoplankton is shown below:
organic + oxygen ---> carbon dioxide + water + hydrogen ion + phosphate + nitrate
Flake fish food is typically about 1% phosphorus (3% phosphate equivalent) by weight (and many products have such phosphorus data on their labels). Consequently, if five grams of flake food is added to a 100-gallon aquarium, there is the potential for the inorganic orthophosphate level to be raised by 0.4 ppm in that SINGLE FEEDING! That fact can be a significant issue for reefkeepers: what do we do with all of that phosphorus? If the food is completely converted into tissue mass, then there will be no excess phosphate. But much of the food that any heterotrophic organism consumes goes to provide energy, leaving a residue of CO2 (carbon dioxide), phosphate and a variety of nitrogen-containing compounds (ammonia, nitrite, nitrate, etc.) as shown above. A fish, whether it is an adult or a growing juvenile, consequently excretes much of the phosphorus that it takes in with its food as phosphate in its waste. Of course, overfeeding will result in more phosphate delivery than will reduced feeding levels.
Unfortunately, many types of seafood available at the grocery store have various inorganic phosphate salts intentionally added to them as preservatives. These foods include canned and frozen seafood, as evidenced by their label, and even some fresh seafood. In these cases, rinsing the food before using it may help to reduce the phosphate load it adds to the aquarium.
Finally, tap water can also be a significant source of phosphate. The tap water that the Massachusetts Water Resources Authority supplies to me is acceptably low in phosphate, or at least it was the last time I measured it. In other water supplies, however, phosphate levels can be too high for reefkeeping. In 2005 New York City officials reported that water samples showed phosphate levels as high as 3 ppm. I'd recommend phosphate testing to anyone with an algae problem who uses tap water, in order to ascertain whether phosphate in the water is an issue.
On the other side of this issue are those reef aquaria without fish. Because phosphorus is required for growing tissue, it is mandatory that some phosphorus source be available to corals and invertebrates growing in a reef aquarium. Finding a source is trivial if fish are in the aquarium, but in aquaria without fish, reefkeepers must somehow add phosphorus. The solution to this problem is easy: either add fish food, even though there are no fish, or add a source of phosphorus such as a plant fertilizer (and if the fertilizer does not contain nitrogen, a source of that may be needed as well).