Cleaning tank

[JF213]

I have a 125g tank with a sump below. I do weekly water changes and have had this setup for a few years now. My nitrates are getting higher faster between my water changes. I have a sponge in the sump that I have been rinsing out. I really think the build up is in the pvc piping that runs between the tanks. How does someone clean this out. There is no way to remove the pvc pipes to do this manually and I know we hate using chemicals.

 

[Delapool]

Can you just flush these out with a hose?

Or increase water changes / any plants possible perhaps.