Do not use soap inside a tank. Depending on what is making it filthy you can use a few things. The best things are those that evaporate. This would include chlorine and ammonia. For calcium type build up a mild acid works well. Vinegar will work but smalls awful. Real acid you need to be very careful using. I have muriatic acid I use for both a swimming pool and some tank stuff. I do not suggest folks with no experience with using acids to try this. Most need to be diluted and if you mess up there is danger both to you and the tank.
I used Oxyclean on a used 150. But I have lots of pumps and hoses which made the job of a thorough rinsing easy. I do summer tanks outside on a screened terrace. They winter out there. In the spring I have to clean them and I take them outside and use a garden hose and a rough pad. I get them very clean. But I also have a small hand cart to move them around and help getting them onto or off the stands.
Without any help from seeding bacteria, it takes 5 - 6 weeks to cycle a tank if you do it right. I would strongly urge you to do a fishless cycle rather than trying to do it with fish. The odds are that you will harm or kill them. A fishless cyle will also let you fully stock the tank right away. Doing a cycle with fish means many months to reach full stocking.
Finally, live plants help a lot with the cycle. Plants use ammonia faster than the bacteria. Also, the plants normally arrive with bacteria on them.
Hint: Most pet store employees have no clue. Pet stores pay very poorly and unless you are lucky enough to find an employee who took the job because the are a fish nut, you will often get bad information.
