With your overstocked tank, you should do water changes, even 20%, every week. All you need is a cup and a bucket. If carrying the full bucket is difficult, carry a half full one and make more trips to dump it out.
A 20% change on a 10 gallon is 16 pounds of water. You could do do three 1 gallon, 8 pound trips to the place where you dump the water. Scoop it out if a siphon is a problem.
Refill the same way.
Do a gravel vacuum once a month.
Changing the water yesterday probably wouldn't have saved the silvertip. Changing it every week for the past 3 weeks might have.
It's a problem that needs a solution. You say you want a larger tank, but if you can't maintain it, it becomes an unwelcome chore for your parents. It's not fair to expect them to do the work if it's your project.