I had a bottle of odourless citric acid based indicator stop open (the concentrate). It has been open for about 6 weeks. I went to mix up some working solution today and noticed strange gelatinous filaments in the solution. I realised I could not use it like that so I phoned the company and spoke to their technical chap. I was told that microbial growth can occur in citric acid stop unlike in stronger acetic acid based concentrates. I was advised that I could carry on using it if I filtered it through a coffee filter. I said thank you, put the phone down, threw the old stop away and opened a fresh bottle.
I think I may switch to acetic acid based stop in future. I only used the citric acid type as I was processing in open trays in a poorly (not) ventilated darkroom. Now I have a slot processor and there is no good reason to be using the odourless stop especially as it seems to go bad quite quickly.
get yourself down to the greengrocer and buy 5 litres of white vinegar. (non-brewed condiment it's sometimes labeled)
should cost about £2.50.
make sure however it doesn't have flavourings added.
I bought some once with pimento.
my pictures got rather spicy for a while