You can help seal and cap in the nutrients with 2 inches of packed down clay, on top of the soil. Around here they sell clay for this purpose at the pond supply store. Your container may not be big enough to support fish very well.

It can take a long time for top soil to settle. Depends on the actual contents.

Water hyacinth can remove some toxins. I don't think I'd go as far as saying they detoxify water. I'm not drinking it anyways. Two struggling water hyacinths, not going to be do much today. Maybe by the end of summer. And muddy water isn't a toxin.

I only use clay soil for water plants. It settles fast, at least some clays do. And that's what lotus grows in so what the heck. If you want more growth you can add fertilizer. I like fruit tree spikes. Cheap and work well.

Top soil isn't exactly what feeds plants, it's the bacteria, fungi and other creatures. They convert stuff in the top soil into plant food. Putting top soil underwater kills most of that system. And a lot of the nutrients that are in the soil will leech out of the water and feed algae.

Other creatures will come in and set up shop, but they're not used to top soil so they might have a hard time. It will become regular pond muck which is good, but that could take maybe a year. Faster to start with clay.

Pieces of mosquito dunks are good for killing mosquito's too.