This is a discussion on Washing within the Beginner Freshwater Aquarium forums, part of the Freshwater Fish and Aquariums category; -->
I have heard that you are not supposed to use anything in a freshwater tank that has been washed wth dish soap. Is that ...
What exactly is the object you want to put in the fish tank?
Soap should never be used on anything that goes into the water, since some objects will absorb soap and it may be impossible to extract all of it. Wood and rock are obvious examples of objects that should never have soap (or any other substance) applied to them, as even boiling wood (don't boil rocks, they can explode) is not totally safe for removing toxins.
Having said that, some ornaments may be OK, but I would give them a very good scrubbing with a brush in hot water. Some plastic materials will absorb chemicals, so caution is needed. "If in doubt, don't" is a good maxim.
Bleach can be used to clean things, but again nothing that is likely to absorb the bleach. And the decor must be thoroughly rinsed in hot water after using bleach.
Anything aquarium-related that I've ever bleached, I've rinsed thoroughly, and then soaked in an incredibly strong solution of dechlorinator for at least twenty-four hours -- and then repeat that process at least once more.
Chemicals, even in minute amounts, can cause all heck to break loose in an aquarium. Typically, I'd recommend against using cleansers of any kind unless it's totally and absolutely unavoidable.
I have two Tetras with ick and I did not want to treat the whole tank. So my idea was to put the two tetras in a small plastic tupperware container with a small filter in it and only treat that water. Should I wash it in bleach?
Also, removing fish is very stressful, and while there are cases where diseased fish should preferably be removed to a treatment tank, it is not something that should be done when there is no direct gain (as with ich) since it will stress the fish further making it more susceptible to ich and other related issues, and could likely kill it esp with sensitive fish like characins (tetras are characins).
The best ich remedy for sensitive fish (which tetras are, along with loaches and corys) in my experience is Aquari-Sol. I have used it with wild-caught characins like pencilfish and wild corys that are both very highly sensitive to any chemical or medication, and never lost a fish but cured the ich (and velvet too). I also do not raise the temperature when using Aquari-Sol, because this again is going to add stress and this has to be weighed against the long-term benefits.
Those shrimp probably won't appreciate the Ich medication. Most Ich medications contain ingredients that aren't kind to invertebrates. If they're not easily replaceable, I'd remove those.
Treating the entire tank is really the only option with Ich. The Ichthyophthirius multifilius parasite can still be present within the aquarium even after the fish have stopped displaying the characteristic outward signs of infestation.
I'd certainly follow Byron's advice and treat everything. Or, to quote Aliens, blast off and nuke the entire site from orbit -- it's the only way to be sure. Heh.
Aside from that, you shouldn't need to scrub anything down. Just grab some medication and follow the label directions. Ich typically isn't hard to kill, if caught early on.