From experience campaigning on behalf of Greenpeace and Friends of the Earth, writing to a body like the RSPCA wouldn't necessarily give the shops selling dyed fish a fright. I believe you need to do 2 things:
1) Pick a chain/large shop, find out the name and address of the managing director/CEO, put up a standard letter on here, and get people to print it out, sign it and send it in. Companies react much more to mail sent directly to them, rather than something from the RSPCA saying "we've had 5,000 complaints". It also needs to go to the Head Office and the top person rather than local managers - this way the person in charge knows the full extent of people's feelings. We also need to say that this is the start of a full blown campaign which will involve the press and other media, where we will advise people to boycott dyed fish sellers, and that we may well be putting up lists of 'bad' shops and possibly demonstrating outside. This will make the company weigh up whether the profits from the dyed fish are more than the business they will lose plus the hassle factor and bad image. We need to overload their mail rooms so they really get the message, and remind them that we are dedicated fish keepers who spend LOTS of money in the industry. Money talks!
2) Set up an on-line petition and information site (in conjunction with Practical Fishkeeping and Death by Dyeing?) and carry out a well targetted campaign, involving all the media attention possible, plus lobbying government etc. Link the information on here with the letter campaigns. Get the general public involved in this side of it.
All this needs to be coordinated and targetted, so that we 'convert' one chain/shop at a time. If we write a few letters to each of the 'bad' shops we won't have any impact, whereas if we all do one shop at a time we will overcome!! We also need to decide if we're doing one campaign, or if we're gonna target UK and USA separately.
I'd be happy to help in any way I can.
Sarah
PS - usual disclaimers, this is just my opinion and if everyone else thinks it's a crappy opinion then I bow to the majority!!