If this were my tank.. Id do a near 90% water change and take the filters out and clean them over the sink. I actually do like a 70% water change in all my tanks weekly anyway, my discus tank I do 70% about every 2-3 days.. If I go beyond 1 week for my tanks my nitrates start to go above 20ppm into the 40ppm range.
Your tap water is 5-10ppm right ? First of all nitrates are bad for humans long term.. You need to either A call the water company or B if you have well water you need a in house filtering system.. They cost a lot but I would never have a house with a well that doesn't have one of those big filtering systems in the basement... Also I wouldn't bother with buying your water.. 5-10 isn't going to hurt your fish and it isn't the reason your tank is at 100PPM.. Your overfeeding your fish and you probably have too many in that tank.
Cut back on feeding too.. how much are you feeding your fish ? A 9g tank you deff should not be feeding your fish a lot. I have a 10g bowfront and I have 4 white clouds, 4 Rasbora tetra and 1 flame gourami and I only feed as much as they can eat on the top and that's it. I only change the water in this tank maybe once every 2 weeks and my nitrates are still only around 20-40ppm.
a 90% water change isn't going to hurt your fish as long as you make sure your temps are exactly the same as you use prime.. I've done 100% once by accident sucked the water to the gravel my fish were flopping on the rocks.. I had walked away when siphoning the water and forgot about it.. Filled it back up and I didn't lose a fish..
