“Will it help rejuvenate my overgrown tank?”
Yes, but not in a way you might be thinking.

Aquatic plants are not perennial.
They are not like trees where you prune and new leaves regrow again and again.

In particular, when stem plants grow tall, the lower portions get shaded and naturally start to deteriorate. The rate of deterioration depends on a few key factors, principally the plant species, growth rates, light intensity, substrate nutrition and overall plant health. In the picture above, the bush of Ludwig pantanal took about 6 weeks to reach this state. The tops (not shown) have reached the water’s surface. Observe the sparse shaded stems, the aerial roots and the poor state of the old leaves.

Above: If we trim away the top of the Ludwig pantanal above, the new shoots that do appear are likely to be small and weak. For the stems that are too decayed, new shoots may not appear at all.

This is because aquatic plants continuously discard old growth and channel their energy towards new leaves. In other words, discarding new growth is wrong. Instead, we discard the old stems and roots (yes the roots) and replant the healthy tops. New roots will sprout quickly. This is especially so for stem plants, which make up 90% of commercially available aquatic plants.

But I see people trim their stem plants?

Oh yes, it is OK to trim once, twice, even three times. But even plants that take trimming well (Rotala rotundifolia for instance) would require replanting after the bottom stems deteriorate naturally over months. Many stem plants do not take trimming well, and should ideally be replanted every time they grow tall – Syngonanthus species for instance.

Above: Stem plants such as Limnophila aromatica above can quickly grow into dense bushes. This causes the bottom sections to be shaded. When denied light, these portions naturally deteriorate after some time. Some species are very resilient and can regrow even from bare stems (such as Limnophila aromatica, Staurogyne repens). However, the large majority of species do better when the fresh tops are replanted.

Healthy tops can be replanted easily. It may feel traumatising to cut and discard the plant’s old stem and roots, but the plant prefers it. Fresh roots sprout easily and happily from healthy tops. Superior nutrition also helps lengthen the longevity of the plant. Growing plants a tad more slowly rather than at breakneck speed lengthens replanting cycles. It makes maintenance easier- you need to replant less often.

Old leaves do not rejuvenate.
Improved conditions show up in new growth, not old leaves.

This is particularly so for stem plants and fast-growing species.
When they get better nutrition, 90% of that boost is channeled towards new growth. In leaves that are still fundamentally healthy, deterioration may slow down, and the leaf may even improve. This is especially so for slow-growing species such as Anubias and Bucephalandra, where some regeneration may take place. But for for most stem plants (and stem plants make up 90% of commonly available plants at aquarium shops), the older leaves do not regenerate.

The picture above illustrates this point. We transfer a Variegated Rotala macrandra from the shop to a 2HR Tank. In the shop, under lower light and inadequate fertilization, the leaves are greener and the internodes are longer. After being planted into a 2Hr Tank, the new leaves produced are fully red, reflecting the higher light and better fertilization. Notice however, that the old leaves that were grown in the shop do not rejuvenate.

It is common for new plants in a new tank to struggle at the beginning.
They may get a bout of nasty algae, as shown on the S. Repens above.

Most hobbyists are delighted when things finally settle down.
Counter-intuitively, the best thing to do then is to cut, discard old stems, leaves and roots, and replant the healthy tops. This is especially so for stem plants. The older stems and leaves that were grown in the shop, or which were damaged by algae during the initial planting do not heal, as shown above. And roots emerge quickly from healthy tops.

So prepare to replant new plants:)