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You may want to try spot treating the nasturtiums with Round-up. If chemical warfare is not your thing, you can also spot treat the invaders with boiling water or white vinegar. Be sure to target just these plants, as all of these methods can kill anything they come into contact with. Don't do it when it's windy and, if necessary, you can shield other plants by covering them up temporarily with a sheet or something while treating and for about an hour or two afterward.

Yellowing leaves can just be the natural aging of the plant.
It also could be watering, either to much or too little. Either extreme can cause the deterioration of the plant.

Inspect the plant for signs of aphids or other insects. A treatment of Neem Oil can be used if you find evidence of infestation.

https://www.gardeningknowhow.com/ornamental/flowers/nasturtium/growing-nasturtiums.htm

This article can help pinpoint the possible issue: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

Yes, it is difficult to see from this image.

It is possible that the seeds were contaminated or the soil.

Always use sterile soilless mix for seed starting.

You can help the germination of Nasturtium seeds by soaking overnight or nicking the seeds prior to planting.
Here is a link with more information.

https://www.gardeningknowhow.com/ornamental/flowers/nasturtium/growing-nasturtiums.htm

Nasturtiums are grown as an annual and will bloom throughout the spring, summer and many times into the fall months.

https://www.gardeningknowhow.com/ornamental/flowers/nasturtium/growing-nasturtiums.htm

The bloom time for nasturtium is mid summer through fall.

For more information on the care of nasturtiums, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/nasturtium/growing-nasturtiums.htm