Well I would use a larger Rubbermaid sterlite container they cost under $10. You feed minimally. I told you min. wc was 25% every other day and you might go daily. This is basically the same routine as is used for an H tank.
I do not think the sterlite is available in the UK though
If you can't remove the fishies then More partial water changes, I just finished an (unintentional) fish in cycle and was having to change out at least 50 % between 2-3 times per day just to keep the levels down.
I tested parameters so much I finished a whole api test kit bottle in 5 weeks
Oh and why can't you use water from the hot tap ? I always do well i have mixer taps, is that wrong ?
What are your results afterwards? I suspect you will need lots of these to make any headway, because as Mr Eagles said, you have a lot of fish for the size of the tank.
In the UK, older style water systems store hot water in a cylinder which is topped up from an open topped cold water tank in the attic. It is this type of system where you shouldn't use the hot tap because of the things that can get into the cold header tank. Eg dead rats.
Hot water from a combi boiler is safe to use.
just did another change today and my friend with an already cycled tank gave me some water and the ammonia went down a little!!! thanks for all your help.
The water generally contains virtually none of the bacteria you need, however the filter media is full of it. Just a pinch or two will dramatically cut down your cycle length.
The issue for copper pipes is not a long term one, It is normally only a concern with new installations. My understanding is after a small amount of time running water through the system the danger goes away.
What I can say is we have copper pipes and our own private well. I have been mixing my water to tank temp directly from the tap using both hot and cold. I have been doing this for well over a decade and I keep 100s of fish and breed some as well.
Many people have mixed reviews on Tetra safe start, but I have used it and know many friends who have and it really helps kickstart the tank. I would change out 90% of water. Fill it up with clean treated water that has sat out for a day( to let the declorinator evap as it will kill TSS product) and add in the TSS. Then you can add in a fish or 2 like the platy.
With your tank being overstocked you can not add all these fish or TSS wont work right.
I would try to rehome the swordtail( not a big enough tank) and possibly danios as they like more room as well. ( 6 danios= 12+in of fish so a 20 gal min for them)
Also cories should be kept in groups of 4+ to be the happiest.