I think you're right that the first tank is overgrown. It's not so much the number of plants but the mass. The plants need to have enough room to continue growing and they've pretty much filled all available space. So it's time to cut them back and thin them out so that they can resume good growth again.
I don't think the second tank is overplanted so much as it needs to be trimmed and have enough space between the stems so that light can reach the bottom. Since the bottoms are looking straggly I'd cut the stems off at or just below the level of the substrate (this will avoid stirring up the soil and the stem may regrow from the base but will most likely rot in the substrate providing nutrients for the replanted stems), remove some from the bottom of the stem and then replant the tops leaving enough room between each stem so that the leaves of adjacent stems aren't overlapping each other completely. You can trim the stems regularly to increase the branching and bushiness. I usually let the plant grow to the surface then cut about a fourth to a third of the way up the stem the first time and then further up each time until it's time to replant. I'd do only a bit of the stems at a time rather than the whole tank at once so not all plants are recovering and getting reestablished at the same time. You might also need to keep the top of the tank less covered with floaters if your stems are stretching for the light and the bottoms are losing leaves. Do you have sunlight that hits the tank at all? I can usually avoid the problem of ratty bases if there is even a bit of time when the tank gets sun or at least bright, indirect light from a window.
I don't think the second tank is overplanted so much as it needs to be trimmed and have enough space between the stems so that light can reach the bottom. Since the bottoms are looking straggly I'd cut the stems off at or just below the level of the substrate (this will avoid stirring up the soil and the stem may regrow from the base but will most likely rot in the substrate providing nutrients for the replanted stems), remove some from the bottom of the stem and then replant the tops leaving enough room between each stem so that the leaves of adjacent stems aren't overlapping each other completely. You can trim the stems regularly to increase the branching and bushiness. I usually let the plant grow to the surface then cut about a fourth to a third of the way up the stem the first time and then further up each time until it's time to replant. I'd do only a bit of the stems at a time rather than the whole tank at once so not all plants are recovering and getting reestablished at the same time. You might also need to keep the top of the tank less covered with floaters if your stems are stretching for the light and the bottoms are losing leaves. Do you have sunlight that hits the tank at all? I can usually avoid the problem of ratty bases if there is even a bit of time when the tank gets sun or at least bright, indirect light from a window.
) and the bases of the plants start decaying. 
