I have been pretty lucky in terms of diseases and parasites in my tanks from day one. I have only seen Ich twice in 24 years and 20-30 tanks running at any one time. However, one of the things I have done is to buy used tanks on occasion. And those I needed both to clean up and also to make safe in terms of any nasties. So I developed a method for how I treated a new used tank.
I set it up in our carport (a garage with few or no walls). I had access to a hose for water and I have pumps to be able to empty tanks. There is also an electrical out let there. Here is my routine.
Clearing out calcium type deposits was step one. I filled the tank with water and added muriatic acid to the tank. This did two things. It removed all but the most stubborn deposits from the glass. But it had another benefit. Adding the acid dropped the pH a lot. Many bacteria cannot survive in acid water. So I may have also been killing some things. After I empty the tank, I deal with the stubborn spots using undiluted Muriaic acid and a Q-tip and then a single edged razor blade to remove those spots.
My next step was the bleach. It has gotten harder to get plain old bleach these days. They are scented or anti-splash. two things I did not want. Many are also "concentrated" which is OK by me. So, I mostly have tp buy the store brand and only the concentrated one and not anything else. Is is cheaper than the name brands. Unfortunately, Covid caused the price of bleach to rise a bunch. I refill the tank with new water and then I add a lot of bleach. Too much is better than too little.
For both the acid and the bleach treatments I let the tank sit for about 15-30 minutes. After I empty the tank from the treatment, I rinse it with clean water while running a pump to empty.at the same time. This lets me rinse the glass well.
The nice thing about using bleach is chlorine evaporates. So, after completing the bleach treatment and rinsing for it, I let the tank dry out completely. Once it has been dry for a while, it is ready to be put into use.
One last observation re cleaning a tank where I am not worried about nasties but do need to clean it up. I had my clowns and redline barbs in a 75 gal. I knew I needed to upgrade them to a 6 ft. tank. I discovered a [erson who was a major Cichlid breeder who was exiting the hobby after back surgery. From him I bout a 150, a 125. a 33L and a 20L all with lids and some with filtration. I built stands for the 6 foot tanks and they sat on the stands for some time without my putting them into use.
One Sunday morning I woke to the in-wall 75 leaking slowly from the from front right corner. I needed to get the 150 cleaned and then move all the contents of the 75 + more to it. I ran to the store and got a big container of OxyClean. I filled the tank, dumped in a ton of the Oxy and put in pumps to circulate the water. After it ran for about 45 minutes, I emptied the tank down to about an inch and began the rinsing process. A bug pump was removing water even as new water come in. At the same time I directed the new water to rinse the sides of the tank.
When I was sure I have over rinsed the tank I began the process of filling it and moving in substrate and all the stuff from the 75. I added an AC 110 and a H.O.T. magnum and moved over the Eheim canister from the 75. I used the media from the AC 70 on the 75 to the 110 and once I had it all running and up to temp, I moved all the fish in. I began the process at about 10 a.m. in the morning and I finished it at about 3:30 a.m. the next morning. I never lost a single fish.
I should state that I did not care to try and figure out how much acid or bleach or Oxy I should add when clewaning new tanks, I just assumed it was safer to add too much than too little. I decided letting things sit with any of the agents I added for longer than I imagined was need. Again, too long beats to short in terms of time. I also felt that the strength of my solutions and the time of exposure was not enough to degrade the silicone seals. The 150 I think was at least 25 years old when I got it used. I have had it running now for at least a decade and it still holds water fine. Some of the silicone that gets onto the glass along the seams has peeled off in placed, But the seals themselves have never let go in any way. The same is true for the 125 which was in better shape than the 150.
Magnum, you have to do things in a way that satisfies you that your tank will be safe to use again when you are done. My feeling is that chlorine can kill just about any living thing when the dose and exposure time is enough or more than enough. I had to buy a lot of gallons of bleach. I figure that 150 actually holds about 135 gallons. To make the bleach I added just 5% of that meant I needed about 7 gallons of bleach. But normally I used even more to be sure it is enough.
When I have to bleach a hospital or Q tank when done with it I also bleach the filters and other things that were in the tank. Depending on the media in the filter I may bleach and reuse it or I may throw it out and replace it. So when I bleach the tank the equipment to be reused is in the the tank when I bleach it. I do not care in such cases that I am killing the good bacteria. With all the tanks I have and with the bottle of Dr. Tim's One and Only I keep in my fridge, I can almost instantly cycle any new tank I gets up. However, when I am not in a hurry, I will dose some ammonia and test to be sure.
Here is what Dr. Tanner of Swiss tropicals has to say about bleaching Poret foam:
Q: how do you disinfect/sterilize Poret® foam?
A: The best solution for disinfection of a fish tank and equipment is still 10% plain chlorine bleach (mix 1 part bleach plus 9 parts water). However, bleach will discolor and destroy the crosslinking in the Poret® foam, so any exposure longer than 10 min (soaking) is not recommended. What works well for disinfection of Poret® foam is simply letting it dry completely (cheap too!). While that does not kill absolutely everything, most fish pathogens will not survive drying. If drying is not an option, use pool acid (sold as muriatic acid this is 20-30% HCl; hydrochloric acid). Dilute the acid with water to about 5%. Add the acid to the water, NOT the other way around, otherwise it can blow up into your face. So mix 1 part acid into ~5 parts water. You can leave the foam in there for hours. It will also destroy algae and most microorganisms. Note that all these disinfection methods do not distinguish good from bad but simply kill microorganisms and algae non-selectively.