As a rule of thumb it’s generally a good idea to thoroughly clean out a second hand fish tank. Despite what a previous owner might tell you about the history of an aquarium it’s impossible to know 100% that the tank is ‘clean’.
A fish tank should have a regular cleaning schedule, be free of sick fish and any disease in the water treated. If the tank hasn’t been used for a long time, even though any waterborn threat might have long since perished there’s always the risk of the tank being exposed to toxins in the air that will pollute the tank.
Additionally you also might want to ‘nuke’ your main tank if there’s been a disease breakout that you can’t get to the bottom of. Sometimes the only way to get rid of a re-occuring disease is to tear an aquarium down and to start again.
These situations require a bit more cleaning power then a simple rinse off with some fresh water. Thankfully whichever product you use the cost is relatively cheap.
Bleach I’ve found is the most effective, probably because it’s the most harsh. Having said that I’d only use a bleach solution to clean an empty fish tank. Although you can rinse off ornaments, driftwood and gravel there’s always a chance some residual bleach will remain and this is disastrous for your fish.
Make sure you never run a filter hooked up to a bleach tank either. The last thing you want is residual bleach absorbed into your filter media or stuck in your filter sponges.
To use bleach to clean an aquarium disconnect any filters and take everything out of the aquarium. Then fill the tank with a bleach solution and let it sit 24 hours.
Empty the solution (you’ll probably want to drain it down a sink then over the garden) and then thoroughly rinse the tank with new water. After that fill it up with fresh water, reconnect the filter and add 3-4 times the amount of dechlorinator you normally use and allow this to run through the system for a few hours.
Drain again and fill up with fresh water treating it with 2-3 times the amount of dechlorinator normally used. At this point it should be safe to add the rest of your equipment, decor and fish back into the tank.
As to how much bleach to use, there’s usually directions for soaking on the bottle (it varies slightly from manufacturer) or as a general rule of thumb a 1:10 water ratio will do the job.
I personally haven’t bothered with Potassium Permanganate but I’ve heard good things about it’s use for cleaning plants and equipment. Being less toxic then Bleach Potassium Permanganate is able to be used to clean aquariums as well as filters. It’s also a little more forgiving then bleach if you don’t thoroughly get all of it rinsed off.
Potassium Permanganate stains pink so be sure to use some throwaway dishwashing gloves when using it and clothes you don’t mind potentially ruining. Also be aware of this staining if you’re going to use it to clean decor and gravel.
For a tank you want to use about 5 tablespoons per 10 gallons (38-40L) and use the same rinse instructions as per bleach above.
Salt is the least toxic of effective materials you can use to clean out a tank. Although it might not be as thorough as bleach or potassium permanganate (this might be a psychological thing though!) salt is pretty effective in concentrated form against parasites and other nasties.
Salt is also safe to use with a filter although be aware it will obliterate your bacteria colony and you will want to remove your sponges as they might be a problem to rinse properly.
To soak an aquarium you want to be using 3 teaspoons of salt per gallon of water (3.8L). Again rinse instructions are the same as bleach but you can skip the overdose of dechlorinator (double the normal dose when filling up with water for the fish at the end though).
In addition to a soak you can also use a concentrated salt solution to scrub the glass if need be. Be aware if you have an acyrlic tank that any scrubbing will probably make your tank look like you rubbed it on some concrete.
Sea, rock and kosher salt are fine to use. Cooking salt should be avoided as well as any salt with additives (usually an anti-caking agent). Aquarium salt is also fine to use but can be expensive for when compared to sea/rock salt which is readily available and cheap.
Totally sterilizing a tank shouldn’t be something you need to do regularly but it should definitely be a thought if you’ve just brought home a second hand tank. At most you’re looking at 24 hours of soak time and a few hours extra work of rinsing/scrubbing.
All three materials are relatively cheap too so it’s worthwhile putting the effort in to ensure the peace of mind that you’re introducing your fish to as clean an environment as possible. Obviously what happens after your fish are in the tank is unavoidable.