Hi
the 50% WC should be enough to sort your ferts excess .High nitrates are dangerous only when they are the smoking gun of previous Ammonia spike that went unnoticed.The once added from ferts are harmless.Its a large tank and your bioload seems not much for it.As overdosing ferts is concerned I was dosing my low tech tanks for long time with full EI and had no shrimps deaths from that.Infertility(struggling to keep the eggs and trowing them constantly) maybe but deaths definitely not..Many on here do high ferts and shrimps with no problems too. I did have few deaths from (dirt)soil substrate going bad and few due to doing large WC(70-80%) with large TDS fluctuations.U need to take the TDS before and after WC.If there is big gap between the 2 figures may be good to do 2 small 25% instead.Cherry shrimps while hardy they still prefer stability .I feed mine spirulina tabs,Hikari shrimp and crab cuisine, almond leaves,mulberry leaves,Blanched (nettle ,kale,spinach...)I tried cuttle bone,mineral stones,calcuim bentonite clay and ...in past to get the colony strong and breeding but seems Im getting slowly there only by being more carefull about the %of water Im changing at a time as current shrimp tank is only tiny (22l )and is easy for things to fluctuate dramatically due to low volume.I dont currently add ferts to the tank as trying alternative metod of growing plants on sand and Californian black worm poop only
In all my other tanks cherries unfortunately got eaten
If U are in my area and your TDS from tap is similar to mine (~160) I imagine your tank should be arround 250-330 (depending on your fert addition)that is in the good range for cherries anyway.U need to get that reading. I should be similar to you then as I live in preston. I feed my shrimp algae tablets, flake, granules. Then they eat all the biofilm in the tank also. Oh and cuttle bone that's in the tank aswel. My bioload isn't big for my tank size. 100/150 cherrys 10 armano shrimp. 3 orange corys 15 neon tetras 20 black tetras and 20 ember tetras. My tank is 400 litres. I will test my water tomorrow before and after the water change and will see the difference.
Thanks lee