What is eating my plant?

I'm fairly new to plants so identification is not my thing. I think the plant in question is wisteria though. Research leads me to believe it's my rainbow fish of which I have only two, but why would they eat it from the bottom up like this? Seems more likely to be a bottom feeder. Would a yoyo loach do this or a hypancistrus? Thanks.

Any suckermouth catfish or snails.

Rainbowfish will eat plants but they start on the edges and try to pull bits off.

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You have a lot of sediment in your tank. Make sure you do regular gravel cleans and water changes.

This tank drives me crazy with sediment, my other tank has nowhere near as much. Believe it or not I vacuumed this tank two days ago. I'd love to know why it accumulates so fast. I've wondered if the yoyos are kicking it up from their constant rooting through the substrate.

Suckermouth catfish like Plecostomus and bristlenose create huge amount of waste and if you have a group of loaches, they will make it worse.

A bigger filter might help pick it up better or just stay with big regular water changes and gravel cleaning.

I knew that about plecos, didnt realise the same was true about loaches though. Thanks for the info. I suppose I should have guessed it since they spend all their time rooting around for food! I'll be moving the plants to my guppy fry tank and I'll be carrying on with the big water changes.

I'm fairly certain the problem with your plant is light/nutrients, not fish. The Vallisneria is not looking good either, which would further suggest this. Stem plants need brighter light in general, and they continually grow toward the light. I insufficient light is getting down to the lower leaves, they will naturally yellow and fall off/melt as the plant grows. The leaves at the top end of the stem are closer to the light so the plant channels its resources into those leaves.

I would need to know the light data and nutrient/fertilizing if any. And tests for pH, GH and nitrate.