Ditto ^
The nitrate climb/creep is something most new hobbyists don't know about (as well as the nitrogen cycle). If your tank produces 20ppm nitrates in a week's time and you do 20% WCs weekly...week 1 - 20ppm, 20% WC brings it down to 16ppm, week - 2 36ppm, after 20% WC 29ppm, week 3- 49 after 20% WC 39ppm etc. Monitor your nitrates and adjust your WC schedule to keep your nitrates under 40ppm.
"No fish died for the first 10 weeks" is a pretty good sign that something happened after 10 weeks, like high nitrates as described above. When things go wrong in our tanks it is usually because something changed and sometimes we don't see a sudden change but the change was happening slowly and finally reached a breaking point.
Swords showing any signs of illness?