Instant water heater issue

So my cousins landlord recently switched & installed 2 instant water heaters.. after a couple weeks noticed the water wasn’t hot enough & loses pressure, started thinking they didn’t install enough heaters for a 15 unit complex. For the past year now they installed 3 filters which helped improve it a bit but now the water pressure drops every 3-4 days and they have to keep washing the filters.. what could be causing this? And this can’t be sustainable or habitable to have to deal with this issue every 3-4 days because people have to shower to go to work & don’t have the luxury to wait for them to wash the filters there has to be a permanent solution? And no their water isn’t hard by any means it averages 50-100ppm