Help needed before I throw in the towel

Hi I have been successfully keeping and breeding several tropical fish for a number of years in a couple of 60litre tank, then most recently I got persuaded to buy a new 150litre tank when looking for a new 60-80litre tank to move some of my growing adults into from my communal main tank....
I also got enticed into trying sand (apparently a easy but step up from my normal gravel based) and also a fully planted aquarium (normally buy potted plants and put in in there pots) anyway....tank cycled fine and I put a couple of mollies in ‘to test the waters’ after a few successful days I went and bought 6 rummy nose tetras, 2 sterbai corys, and 2 Dalmation mollies....all has been fine for the week since I purchased then last night upon getting ready to go to bed i decided to move a couple of the adult Male guppys to this 150litre tank (5 to be precise)
Upon waking this morning all 6 rummy nose tetras are dead (bellies look like exploded as have some hanging out??) and one of the mollies is dead! It’s a fully planted sand based 150litre aquarium which only had 6 rummy nose tetra, 2 sterbai corys, 2 dalmation mollies, 5 Male guppys, 3 other mollies and 2 cherry barb in? Why did they die? Water was also slightly cloudy this morning? But hasn’t been for the two - three months tank been running? I’ve now took all surviving fish out (except the corys as couldn’t catch them) and put back in the 60litre and they are happy? I can’t test the water parameters at present as lent my kit to a friend and haven’t got it back yet but I just don’t understand??!? My daughter is devastated as she loved the rummy nose tetras and picked them herself and in several years of fish keeping we have lost a handful of fish in total never 7 in one night?!? Please help? I’ve just done a 50% water change but don’t know what else to do or why they died or what’s wrong??

We will need more data to help. You said the new tank was cycled...could you explain how you cycled it? And are there live plants?

If you have tests for ammonia, nitrite, nitrate and pH, it would help to know these numbers.

What is the hardness of your tap water? General or total hardness, you may be able to get this from the water authority (website, or call them) if you don't already know. You have fish with very different requirements in GH and pH.

How long has the 150 litre tank been running for, and when did you first add fish to it?

I assume there is a filter running on the tank and it is run continuously?
When did you last clean the filter and how did you clean it?

What is the temperature of the aquarium water?

When did you last do a water change (besides the 50% today), and how much do you normally change?
Did you dechlorinate the water before adding it to the tank?
Do you normally gravel clean the aquarium when you do water changes?

When did you feed them, last night before or after adding the 5 guppies?

If the water is a milky cloudy colour then that is a bacterial bloom caused by uneaten food and or dead fish. Milky cloudy usually means there will be an ammonia reading. And if the water smells bad, then there is ammonia in it.

Can you post a picture of the sick or dead fish?

That's terrible! I'm so sorry.

Where did you get the plants and sand from? Did you wash the plants and sand prior to cycling the tank?

The tank may be cycled but there still could be parasites living in it. Although I've never heard of a parasite killing 7 fish in a day.

I know you did a 50% water change..... I'd do it again.... and again. If there wasn't an ammonia spike before, 7 dead fish caused one for sure.

Ok, I don’t have access to test kit at present as explained in post
It was planted out, filled up and left running for 4 weeks empty....then added the mollies to test the water, after approximately a week maybe 10 days of just having the mollies in there I bought the other fish which had been in tank for approximately 10-14 days.
Yes they are live plants and lots of them!

The tank has been running for just under 3 months now (1 month empty, 2 months nearly with fish in)
Temperature is 27
Yes there is a filter running continuously, and was last cleaned (other than today) 2 days ago in a little tank water?
Last water change was approximately a week ago and only roughly 20%
I haven’t done a gravel clean in this tank yet as there is no debris whatsoever (maybe because it was very low stocked and I don’t over feed?)
New Water is always dechlorinated with fluval aqua safe plus water conditioner.

Last fed flake yesterday morning, then last night a couple of Cory pellets dropped in before adding the guppies but we’re gone by this morning.

Dead fish gone and no sick fish to take photos of?

Photo of the corys last night when I fed them before adding the guppys (because I was going to ask today whether they were Male or female lol)

Pics won’t post?

Files probably too big. Pics from my phone do that. I have to edit and resize them before the site will let me post. Maybe you have to resize your pics, too.

You really do need your test kit back to run a cycle. You don't mention adding ammonia. Without a source of nutrients the neccessary bacteria won't develop so the tank may not be cycled and/or you may have increased the bio-load too quickly. Adding 10 fish at once when you have only had 2 is a massive jump. Then adding another 5 was another big jump (as well as the extra food).

Without the actual test results we can only guess, but my guess is that @IHaveADogToo is right and you have had a massive ammonia spike, and you certainly do now. I would do daily 50-75% water changes and only feed sparingly every 2-3 days until you have the test kit to figure out exactly what is going on - and when you do feed change the water within a couple of hours.

If the other tank is still running you could take its filter sponges and rinse them out in a clean bucket of fresh de-chlorinated water and tip that into the tank (don't rinse in the tank itself in case there is an infection that could transfer back to your old tank). It will make the water mucky for a few hours but it is the most effective way of getting the good bacteria across.

Picture of tank I just took

Ceri84 said:

View attachment 87589 Picture of tank I just took

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That looks very good. Hope you manage to get it sorted soon.

seangee said:

That looks very good. Hope you manage to get it sorted soon.

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Thanks I was really looking forward to my new venture and now beginning to dread it just so disappointing and now I don’t know whether to chase the corys and remove them or leave them in there or what to do but scared of putting anything in there to be honest

Oh my plants were the same price or equivalent as gold! from world of mosses
My sand was from swallows and was washed very thoroughly in fact probably too thoroughly as the muck that came off it was disgusting apparently normal for sand

I know how hard it is catch corys. The struggle is real. You did just reduce the bioload considerably, though. If you are doing daily water changes until you get the water's chemistry figured out, they might be okay. Still a gamble, though.