Carbon in the filter is only necessary to remove medications after treating for disease. Changing it may help reduce the odor, but it's an expensive way to do it. Are you siphoning the gravel when you do partials?
I noticed your 2.5 gallon tank is your hospital tank. What are you keeping the filter alive with? Do you currently have a fish in there? If it has been fishless for a long period of time the agent you are using to feed it may be the problem. I just keep an extra filter in one of my main tanks so I can set up a hospital tank when I need one rather than try to keep an empty tank running all the time. As far as the 20 gallon goes, I think the gravel vacuuming may help cut down on the odor some, but what kind of filter are you using? Also do you have a brown algae problem? I would also do at least a 50% water change (a big job I know, but I consider that a minimum for this kind of problem) (Do the gravel vacuuming at the same time as it may stir up quite a bit of debris if it hasn't been done for a while)
I did have a brown algae problem, that's why I got some snails.....they took care of that over night.....do you think that the algae fot into my filter and started to rot? cause it smells pretty bad.....
If your filter has a sponge in it, squeeze it out in old tank water. You can also swish your filter media in a small bucket of water to get some of the crud off and help with the flow if it's sluggish. That might help some. Are your tanks cycled?
well I changed my carbon filter (I left my bio filter in) and did a 35% waterchange while I vaccumed a bit...and the smell has subsided for a bit...it's only been a couple days though so we'll see in a week.