I guess haven't seen nor heard of any fire caused by tank filters for the past few years but nevertheless the following are the safety precautions that you can take.
1) Firstly, before you perform your weekly maintenance/ change of water for your fish tank, always ensure that your filter is turned off first.
This is to prevent being electrocuted in the event if there is an insulation breakdown in the filter.
[ I suspect most filters used double or reinforced insulations as they only have two pins plug(without the earth connection). ]
2) Secondly, look for a reputable brand like Aquaclear, Eheim, Fluval, Aquael, GEX (Japanese brand), etc when buying a filter.
3) Thirdly, if you want to be even safer, look for filter that have the safety marks.
For example, JBL brand has a German safety mark, "TUV-GS" which means it has been tested and approved/certified for safety.
Here is a link for JBL filter:
Aquaclear brand seems to have a Canadian safety mark "CSA".
Some brands may have CE (European safety mark). But CE mark is a "self-declare" safety marks by the manufacturer, unlike TUV-GS or CSA which are certified by a third party agency.
So, a CE mark alone without a third party certification mark may not necessarily guarantee the safety of the products and the users.
I've seen some very cheap products that have CE marks and I doubted their safety.
From my many years of working in design and product safety, we know that it's costly to produce a product that complied with safety/regulatory requirements and it may not be cheap when you send your product for safety certifications.
Risks of fire
In Japan, the 100Vac supply may not necessarily be safer as the risks of fire depend on the power available to the filter(or any product) and not just the voltage.
I supposed that in Japan, the circuit breaker rating is 20A or 30A for the house.
So, if the circuit breaker is rated 20A, the total power available is 100V x 20A = 2000W which can easily burn anything.
But if you used an extension cable which usually has a 13A fuse, your available power is 100V x 13A = 1300W which is still very high and can easily cause fire if there is any short circuit.
Risks of electric shocks.
But on the other hand, 100Vac has slightly lower risks of electric shocks as compare to other countries that have supply voltage of 230/240Vac.
[ The risk of electric shocks is dependant upon the supply voltage. ]
Nevertheless, any voltage above 30Vac is consider dangerous and can cause electric shock. So, it may make not much difference between 100Vac and 230/240Vac since both are consider hazardous voltage.
Solution: Anyway, usually you don't have to worry about the risks of fire or electric shocks caused by "bad designs or bad material used" if your filter has a safety mark(third party certification).
Other suggestions:
4) If you are considering installing a separate circuit breaker or fuse, you have to ensure that the current rating of the circuit breaker is low enough to prevent a fire.
According to some safety standards,
your power limit should be less than 100W to prevent fire.
It means for 100Vac supply, you need a circuit breaker rated at about 1A. If your current rating is more than 1A, then there are risks of fire.
But if you are sharing the circuit breaker for a few equipment, then you have to use higher current rating of circuit breakers.
But then, your circuit breaker may not achieved it's purpose. In this case, you may have to dedicate a single circuit breaker for each equipment in order to be effective.
Solution: This may not be necessary if your filter has a safety mark unless you want to have double precautions.
5)Provide a cover for your power socket to prevent water from getting in.(very serious as it can cause fire).
This is required only if your power socket is directly below your fish tank.
If your power socket is at the side and some distance away, or above your fish tank where the water from the tank can't get into it, then you don't really need this.
