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The Marine or SW rules on Copper apply with FW also. You are correct in your theory that it can kill inverts, its not an always gonna kill, but for the most part it will. Also, once its in the system, its in the system. It gets into the glass, and its a long process to get it out. But, if keeping the systems seperate, there shouldn't be an issue with reguards to fish and inverts, but if in the same tank, theres gonna be an issues. Also, people who buy fish from LFS should never throw the fish and the water from there into a DT, bad idea, with bad problems arising from that. People should always dump out the water and just put the fish in the DT.
Coppe Sulfate is the bad boy of Coppers for Inverts.
Many of us know the story first hand. You get the new, exotic plant that you were looking for to embellish your freshwater aquarium and bring it home. After a good rinse, you chose the best spot for it and plant it. A few days later you find a snail… and a few weeks later there are many, many more…
There are many ways to treat a snail infestation which are very well described elsewhere on the internet. However, here I would like to discuss the use of the chemical known as copper sulfate pentahydrate (CuSO4•5H2O). Copper sulfate is a compound that has been used for a number of plague treatments. It is an algaecide, anti-bacterial, anti-fungal, works on external protozoa such as ich and serves many other purposes too. However, it is also toxic to many other living beings and that includes other invertebrates, plants and fish.
Yes, copper sulfate is very highly toxic to invertebrates such as crab, shrimp and oysters. If you have any of these in your freshwater aquarium, you should not use copper sulfate to treat a snail infestation because they will die too.
Yes, copper sulfate is toxic to plants. It disturbs photosynthesis and can kill some varieties. Nonetheless, some species are more resistant towards copper than others. Both Vallisneria sp. and Sagitaria sp. are known to be very sensitive to copper, but Anubias sp. and most mosses will survive a treatment with copper at the recommended doses.
Yes, copper sulfate is toxic to fish. That being said, however, at the correct concentrations, CuSO4 is safe to use in a community freshwater aquarium provided it is used correctly. I, myself have used in my tank without any losses of fish and, though I agree that it is risky, I also believe that a treatment with copper sulfate, if done properly, can be used to greatly reduce a snail infestation.
Vertebrates, like fish, are much more resistant to moderate concentrations of copper ions than invertebrates like snails. Snails are extremely sensitive to copper sulfate. In fact, concentrations as low as 0.01 % kill all the snails present in any body of water in less than two hours. Moreover, recent research carried out by the National Warmwater Aquaculture Center in Stoneville, MS, showed that concentrations of up to five parts per million (5 ppm) of copper sulfate would not kill catfish in outdoor ponds while killing more than 90% of the snail Bolbophorus sp. These concentrations, however, are much too high for a typical freshwater aquarium and should not be used.
It is well known that the lethal dose 96hr-LC50(20C) for pond snails is 0.39 mg/L (or 0.39 ppm). This means that a concentration of 0.39 parts per million of copper sulfate in your tank will kill half of the snails present over a period of 96 hours if the temperature is kept constant at 20 ⁰C (68 ⁰F). This number is a good place to start. This concentration will not harm any healthy fish, but will kill half of the snails present. Pretty good, isn’t it?
Now, if we double the dose, will the massive death of snails occur faster? Will it be a larger die-off? The answer is, probably both; however, this is not recommended at all, since a larger dose will become much more toxic to the other organisms in the tank than two normal doses at different times, when most of the copper has already been removed from the system (more on that later). Always remember that there is not a normal linear relationship between the dose and the effect. A double dose will have a toxic effect much larger than twice a normal dose. Doubling the dose is, therefore, greatly discouraged. At this point it is important to make it clear that, in very large systems, the use of low copper concentrations maintained for a longer period of time might be more effective to control a snail infestation than a higher concentration for a shorter period of time.
It is usually agreed that safe concentrations to use in a typical freshwater aquarium are in the range of 0.15 to 0.2 ppm (or 0.15 to 0.2 mg/L). Any higher may leave the fish with red sores on their sides. However, these numbers have to be taken with care because there are other factors that influence the toxicity of copper ions: The temperature, the alkalinity and the hardness of the water. Copper is much more toxic at higher temperatures than it is at lower temperatures. In fact, for some species of snails, the toxicity of copper sulfate (measured as mortality) increases more than four times when increasing the temperature from 15 °C to 20 °C (59 °F to 68 °F). If you are treating your tank with copper sulfate, try to maintain the temperature at about 22 °C or 23 °C.
The other factors that greatly influence the toxicity of copper ions are the alkalinity and the hardness of the water. In water with a high pH (high alkalinity) and high hardness, the copper ions are sequestered (inactivated) and the effectiveness of the treatment will be reduced. On the other hand, if your tank has very soft, acidic water, the toxicity will be enhanced and many people discourage the use of treatments with copper in these tanks because of the great risks associated with it. In my opinion, however, if you are determined to use copper to treat a tank with soft, acidic water, it is best to start off with a very small dose (about half of the recommended dose) and go from there.