I've looked around for an answer on several forums in my native language and around the web, but I could not find the answer. Maybe one of you could help! Thank you for reading.
I have 2 aquariums (450 and 105 L) and I got my gf a 20L hexagon aquarium equipped with decent light for non-demanding flowers, there is no heater and the filter is simple sponge filter of according size connected to sera 275 air pump (both ends) filter creates decent water rippling and thus aeriation.
I bought 7 CPO from a person in my city, they all seemed healthy various sizes from 2 cm to 3 and one had 4 cm. As I knew that the tank is too small for this many I had bought them with the intention of putting the rest in my tanks and keep 2-3 in the 20L which would be in care of my gf..
To create more hiding places during the period when 5+ were in the small tank (week?) I had bought a bamboo stick of appropriate diameter and constructed a hideout. We put away the biggest CPO to my 450L(120gal) tank and added a m8 for him that was around 3 cm later on.
The tank now had 5 CPO in it and problems arised, 2 CPO died day after another, 2.5 cm size ish good color, no wounds, they died on their back and they were still sorta moving their legs for a while, but they exited few hours later.
Now about water care.. water quality in all of my tanks has 5-7°d GH (7 for the CPO tank) and around 5-6°KH, PH is around 6.8-7 pretty constant, no chlor or choramin.. I'm using sera aqua safe on all the water I add to the tanks. NO2 is at 0 and NO3 is up to 25 before the water change, water changes were done on weekly basis before the first death, after that I did them every 3-4 days, amount of water changed is roughly 30 to 45%, before the deaths we had a steady 26°C temperature, but afterwards we changed it to 22-24°C and even in the hot days it was like that. I had the tank cycling for 2 weeks before we put them in, but it has been over month and half now.. and from those 5 CPO only one remains in the small tank.. and from those 2 in the big tank the 4cm one lives. Basically from those 7, the biggest CPO lives in my aquarium.. and in hers the smallest one which had his pincers gone sometime before we bought him survived, all the other good looking healthy and bigger CPO's died in the same way, on their back. I've asked several people from my country and even some sellers.. but I have not received any help. I forgot to write aboud feeding, we feed the CPO in the small tank with Hikari pellets made for crayfish and the crayfish in the big tank gets regular fish pellets of various types. Water in the tank is clear, nothing rots there, water is changed frequently.. she is very sad when one of them dies.. and this last one alive is her favorite.. because he had no pincers (one is growing back at the moment).
What could be the cause? I'm pretty serious about these things and I try to do it "by to book" as far as I can, but they still die. Whenver I read about similar deaths.. either people do not know or just vaguely guess, haven't had any specific info so far, I'm trying to keep the water as clean as I can, no lefover food rotting anywhere or plants.
I'm adding a photo of the small tank, it has brown substrate (bought in aquarium shop, cleaned it for 2 hours straight before putting it there) and another photo is of the 4cm one from my 450L tank.
Thank you for your time in reading and maybe someone has an idea what to do to keep the last two alive.
