Well, thats a sign that the algae is doing its thing and fixing some nitrates. Man, you guys must have a lot of waste around the tank...
Dunno if the LFS can test the TDS of your water, but if they cant, perhaps someone on this forum might be local enough to you (and have a TDS meter) to bring a sample to them and have it checked out. A TDS meter is the only way to truly know if your RO membrane needs replacing. For example, my RO membrane puts out water with a TDS of 2ppm despite being over 2 years old. It functions fantastically cause I flush it very often. 2ppm means that only 2 parts per million has any solute other than H2O... A very good reading. 10 or less is generally an acceptable RO discharge TDS. If you're over 10ppm, flush the membrane. And if after flushing its still above 10ppm, the membrane needs replacing (and you might also want to examine any pre-filters at that time to see if they need replacing. TDS meters are pretty cheap in the stats but I dont know what they are in the UK. Here they're like $15 online, dunno about there though
Dunno if the LFS can test the TDS of your water, but if they cant, perhaps someone on this forum might be local enough to you (and have a TDS meter) to bring a sample to them and have it checked out. A TDS meter is the only way to truly know if your RO membrane needs replacing. For example, my RO membrane puts out water with a TDS of 2ppm despite being over 2 years old. It functions fantastically cause I flush it very often. 2ppm means that only 2 parts per million has any solute other than H2O... A very good reading. 10 or less is generally an acceptable RO discharge TDS. If you're over 10ppm, flush the membrane. And if after flushing its still above 10ppm, the membrane needs replacing (and you might also want to examine any pre-filters at that time to see if they need replacing. TDS meters are pretty cheap in the stats but I dont know what they are in the UK. Here they're like $15 online, dunno about there though