Conductivity, when to do WC
This whole EC-thing is pretty new to me, so I apologize if I sound like an idiot. I bought an EC-meter. Specs of tap water are GH 4-5 (now, changes quite alot between 4-7), pH now 7,4 (usually 8), KH3 (-5), EC ~620 (microSiemens/cm, oh my, this high was a bit surprise!). Aquariums (four) GH7, KH3, pH 7-7,2 EC 750-850 (11 days after last WC 30%). NO3 and PO4 are really low now, I use a bit some ferz (K, Plantex, Fe, Equilibruim and Excell daily). Tanks are full-mid planted low-techs, mid stocked I suppose. GH keeps going down ~1dh per week so I must add it a bit regularly.
The question is: when to change water? How much I can let EC to raise before WC? 100 microsiemens, two? I have tried really hard to find the answer from net without luck. NO3 is so low it cant be the factor, EC keeps going up from day to day so it must be that.
For sure the water is not very suitable for soft water fish species wich we also have (Amazonian and Asian species) among bows and blue-eyes but fish has done well so Im not sooo worried about the tap water. On the other hand Id like to keep the water as clean as posible in this situation when the water is not so ideal for all fish. Im considering to give away soft water fishes but since they have been fine, maybe just not to buy them anymore, just hard water species? Other specs than EC are fine for soft water species (in their own tanks) and med-hard water species (in their own tanks) but the conductivity is hardly OK or med-hard water species! Confusing!