The short answer to your question is probably no, you're not at any great risk. Assuming you use the dosage recommended, the amount of the carcinogen (likely formalin) in the water will be so small that any risk will be essentially trivial. In other words, no worse than anything else you might have done today like eaten barbecued meat or French fries (carbon carcinogens), eaten bacon or other smoked foods (nitrites), had an x-ray, or walked along a street where you could inhale fumes from a diesel motor vehicles.
There are risks from fish tanks, but cancer isn't really one of them. Electric shocks are probably the most common life-threatening dangers. Broken heaters expose mains voltage to the water. Pull a plug from a wall socket with wet hands and you can get a shock from that, too. Some aquarists recommend working in fish tanks only after disconnecting all appliances from the power. My little Mirabello tank actually says as much on the hood!
Salmonella is another potential risk. These bacteria can live in aquaria quite happily, especially dirty tanks. Any contact between the water and the mouth is a potential risk. It's a good idea to wash your hands after working in the tank. A healthy person probably isn't at much risk at all, but those with compromised immune systems should almost certainly take this risk seriously.
The fish are another possible risk. Lots of people get stung by things like lionfish. The strength of the venom varies, and virtually all the fish traded as pets come under the heading of "painful" rather than "lethal"; the one exception is probably the stonefish. While lionfish and stonefish are known for their stings, some aquarists are less familiar with the fact many other fish have been reported as being potentially venomous. Many catfish are venomous, and even the non-venomous ones like talking cats and Corydoras seem to have barbed spines and some sort of mucous that causes inflammation in humans. Scats are another group of fish that appear to be venomous, at least in some circumstances.
Bites should always be taken seriously, but morays in particular seem to have nasty mucous in their mouth that causes real problems in humans. Anyone bitten or stung by a fish should get medical attention. It is extremely important to write down the Latin name of any dangerous fish being kept. Medical treatment depends upon correct identification, and common names are too misleading and varied to be helpful. That said, most fish venoms can be dealt with effectively by placing the wound into piping hot water. The heat denatures the venom protein, reducing the reaction (and, eventually, the pain).
I'm not an MD, so my opinions are posted here purely for discussion purposes and shouldn't be treated as medical advice under any circumstances. If you have concerns about how your fish tank affects your health, talk with a medical practitioner.
Cheers, Neale
