I have a couple of plastic fish tanks that have been lying since the DDs fish expired about a year ago 
. They still have the old gravel in them and I keep meaning to get round to cleaning them and trying to sell them (we have "done" the goldfish thing now). They're not very big - one big enough for 2 small fish and one a secondary "holding" tank meant to be used when you're cleaning the other one. Can I use bleach and/or washing up liquid for cleaning or will it "taint" the plastic and harm any future fish? What would you use for cleaning these?