fish are dying

[weedkiller]

all the fish in my 64L have all started acting strange, and swimming abnormally, one of the black skirt tetras died this morning, no visual signs of anything, then one of the corys this evening, looked a bit red underneath, another tetras just gone no visuals, the gouarmi looks to be soon with another tetra, that will leave one tetra and one cory, the remaining cory seems to be shooting up for air more often (do have an airstone), has no redness like his buddy did.
dont expect any to be around come morning, shame as it was a nice little tank.
will leave this tank for now and wait till my new fluval 406 has finished cycling with my old fluval 206 and put the 206 on the smaller tank and restock with something different

[endlerguppy]

do a water change and test the water

[weedkiller]

have bud, did a 40% change earlier and parameters fine except nitrates a bit high but they are used to that, this is why i cant wait to get the 206 filter on it and i can have some matrix filter media to get them down, since i started the post another tetra has gone
is amazing how quick it can all go wrong and for what appears no apparent reason

[weedkiller]

woke this morning to no more tetras, the corys on its side breathing, no redness like his bud had and the gourami is hanging in but im sure wont make it.
that is just crazy at how quick it happened, the only thing added was food and a water change after the 1st one died.
just been and checked on by 190L and all looks fine, with the exception of the mega clown pleco everyone at the tank to say 'hello wheres my food'

[emc7]

run basic checks, pH, temp, hardness, CO2 whatever tests you have. Check for electrocution. A lot of fish at once is usually the water.

In a small tank, usually ammonia or nitrite, but it can be anything toxic:food, gas,
a "bad water change" which can be low Oxygen, forgotten dechlor, way-off water parameters (sometimes water systems OD choramine or NaOH). A CO2 system can go nuts, HS can bubble up from a dense substrate. Its really amazing fish live in tanks at all.

[weedkiller]

i use same water for both my tanks, all parameters were fine, did another 20L r.o water change today in a last ditch effort to maybe give the last 2 a chance, came home from shops aftera few hours to find the last cory dead and the gourami suffering so i had to put an end to his misery... shame.
gonna wait till my new filter has finished cycling, been 1 week so far then i will add my old filter to this tank and start again, was thinking of starting a breeder/feeder tank so i got some live food for my acaras, got the missus saying i cant because its cruel lol, cant win.

[MainelyFish]

That really sucks sorry to hear this happened....