Only in one instance in the case of fish can I say could be toxic. I will talk more on this later. Other wise it is safe. To inverts yes it can be toxic. Now I have not tested this on prime I tested this on erase-cl brand. Which I think to have identical "proprietary" ingredients. Using erase-cl brand I think I have identified the proprietary ingredients in this with some legit testing. I can show you how to test this with yours. I will also explain why it is not as toxic to fish but as you see elsewhere it is toxic to inverts. Now for the fun stuff. First test ph. Second find density. Third find boiling point(good way not with thermometer based on feel). Fourth heat to dryness. Fifth heat past dryness to near decomposition. Sixth flame test observed with the iconic color of sodium flame. Seventh heat to decomposition. Some of this may be left out or lost in translation. I will mention what I observed along the way. Crystal clear solution with ph 6 acidic "proprietary" salts listed as a mixture of bisulfites, and sulfonates. (for me that was the give away) Density 1.2588 g/mL average. Boiling point very low I could touch the metal briefly it was around 200 degrees F. Heat to dryness no smell. Heating past this gives you some more clues. Working with sodium bisulfite (acidic) and other sulfur compounds in the past I know the smell of sulfur di and trioxides it is unmistakable. This happens starting around its boiling point. It also melts at a low temperature. Turns into an amorphous solid when all these ingredients are mixed and melted. The amorphous polymer picture is typical of melted plastics, or polymers. It is also able to dissolve again, but very slowly that tells you it is typical of something that is water soluble, and considered amorphous. Also you can see but not make out as good in person carbon scoring in the picture residue after, and some before decomposition. As well see carbon well in the residuetest2pic. This again is typical of organic compounds if heated slow enough. See in residue in water after picture it is hard to make out there is green substance dissolving. Now because the bisulfite released sulfur di/trioxides It reacted. Creating green Iron sulfate salt. Also you see stainless steel spatula after washing with iron oxide stains. I would of heated and ph tested if my torch could go high enough. (not really necessary). Now if you look at all these clues, the density especially, and match the characteristics of this test. All matched to the properties of said chemicals. This to me matches Sodium bisulfite, and vinylsulfonic acid or vinylsulfonic acid sodium salt. In other thread I can provide reasons why their tests were flawed. Firstly they showed test that it showed no nitrite. Why? Sodium bisulfite will react with nitrite making nitrate. This will not read on that test kit as shown there. Secondly you see them suggest it is a thiolsulfate salt. Well its not even really dilute solutions have a tint of yellow color. Thirdly you see them test with seneye and test strips. They claim no drop in bound measurable free ammonia. Why? The seneye, and strips are still picking up the ammonia molecule. I challenge them to test with another source of bound ammonia. Like maybe trimethylamine solution, urea salts, or amino acids. A lot of those can trick tests. I would bet it would still pick up free ammonia. Nor did they ever test ph of solution they were testing. Likely the ph if they tested was not much more or less than seawater they started with making ammonium the dominant species anyway. I do believe it does measure ammonium but conversions need to be applied. I forget where I saw this for reference. Ammonium would still get bound. Then I'm sure someone will say no only ammonia is toxic. That is just not true either. Whether it is a combined test kit, or solely monitoring ammonia. It is majority ammonium at tank ph. If it goes to over .5 -1 mg/L even you stand a good chance of losing fish. Now lets go back to prime. In my opinion it is toxic to inverts, as well as fish really not to the point of death if you keep it reasonable, and don't just count on it. The way it works is that it binds the ammonia/ammonium compound, making them get into the fish at a much slower rate. This is why you see test kits going up, and up over time. As well as it being harder to be biodegraded by bacteria because it is a more strongly bound and bigger compound than their used to. Why is it toxic to inverts? Simply this at whatever rate it gets absorbed into their bodies is unknown, but you can see in their thread it does kill them. It is because of one of the ingredients! Vinylsulfonic acid complexes get into their blood stream faster. As most know inverts are more prone to osmotic shock. As well it reacts differently with their blood serum. It is a odd mix of amino acids, free floating hemocyanin, histamines, and others. Vinylsulfonic acid can react with compounds, and others internally. Creating a few different amino acids, free ammonia possibly, and even possibly free copper. Hemocyanin is well known for its catalytic activity. This all happens quickly now can you imagine why it kills them? To mention there is also other chemicals, and blood types in our tanks. Hemoglobin, chlorocruorin, and haemerythrin. All having different reactions but likely not as bad in fish simply because it does not get inside them nearly as fast. Maybe the molecule is to big to even pass through their skin, or gills. Maybe it is like chloride competition with nitrite. Maybe it is because hemoglobin is bound more strongly in a protein versus some inverts. I wont get into this anymore (its really complicated) It would really take a few more pages. My point though. Only ever in two situations would I not use these. As you can see it does not harm fish if used responsibly hundreds of thousands of people will tell you that. 1: if you had inverts to worry about just move them to another tank, or freshly mixed salt in a bucket if you have to. 2: in a quarantine setting with copper medicines. Why? It makes some nasty ammonia complexes with copper. That in it self is a whole other can of worms to open.