so my 10 gallon dirted tank has been up and running for about a year. The water wisteria was doing great at first but then seemed to get stunted and grew much slower. Why is this? (No ferts, co2. Just dirt)
Less fish food going into the tank, producing less ammonia?
More plants using what little food there is.
Less light due to more plants competing for it, and older fluorescent globes producing less light?
If you are feeding the fish each day, there should be some nutrients (ammonia) produced each day. A filter might be removing most of the ammonia and the plants will have to use the nitrates instead of ammonia.
As for the gunk, it will be getting topped up each day when the fish is fed or goes to the toilet. There isn't going to be much because there is only 1 fish but it will always be there.
If you only have 1 small fish in a well planted tank, the plants will always be hungry for nutrients, and Hygrophilas are pretty hungry plants that love nutrients, so you will struggle to keep the nutrients up for the plants by only feeding one fish. You could try feeding more often or add a liquid iron based fertiliser to give the plants a bit more to eat. I use Sera Florena but there are a number of different products that do a similar job. Iron is the main thing to add to an aquarium to give the plants a boost and they don't normally need carbon or nitrogen supplements because they can get these from the air and fish.
If you don't want to add supplements then don't worry about it. The plants look pretty good as it is.
The leaf shape of Hygrophila difformis (Wisteria) is determined by the temperature and light. At lower temperatures the leaves are smooth-edged and smaller and less pinnate; at higher temperatures and brighter light the leaves will be sharply divided (pinnate) and closer together on the stem. Lower light will also cause the lower leaves to fall off.
Being a relatively fast growing plant, it needs good light and nutrients. Soil is rarely going to provide sufficient nutrients. The only advantage of soil is the organic decomposition which occurs immediately, thus producing CO2 early on. Inert substrate tanks take a few months to get to the same level of organics in the substrate. And what nutrients might have been in the soil initially, if it was a specific plant enriched substrate containing nutrients, will have likely been exhausted by one year.
Ammonia as Colin said is also going to be lacking here. Ammonia occurs primarily from fish respiration and the breakdown of organics. With only one fish, and the organics largely exhausted, this is likely one nutrient that is now minimal.
Its a heavy feeder. You can go longest in big aquariums with it because its constantly growing new stems and spreading. In small tanks,slower growth and stems age. Sometimes you just need to take new cuttings from the top and plant those next to the old stems..and trim the old stems far back and hope most rejuvenate.
I never use Co2 or fertilizers. Rules are different with them.
The temperature range usually given for this plant is 23-28C/74-82F. At the lower end the leaves will be smaller, smooth-edged and less pinnate, and at higher temperatures and brighter light the leaves will be sharply divided (pinnate) and closer together. Light also factors into this, naturally.
I agree with Byron. I've always prefered 78F-80F for best plant growing ( in gravel NOT sands) and fish keeping. It's what they see in the tropics. Maybe subtropics and microclimates like dense forest lowland streams run cooler,but that's where you have to know the habitat.
Still,Crypts for me grew fastest at 80F..and all bunch plants love that and lots of light.