Excess slime coat is produced to protect the fish if/when water is not suitable for them. It sounds like, too many fish too soon as violet has said.
Have a read through the link under my signature strip on the nitrogen cycle; this will explain more about water quality and the need for it.
A good water test kit is an essential piece of kit for any fish keeper, shops, 99% of the time will tell you your water is fine, for healthy happy fish you need to check this for yourself and your own piece of mind.
Oppsy..clashed with Noodle.
PS if you can get your water tested (ASAP) again at your LFS ask them to write the results down so you can check them on here.
Do you use tap water or pond water to flush the pump and clean the sponges?
Your filter really shouldn?t need such a thorough clean, i would suggest once a month-6 weeks should be about right, using pond water only as the chlorine and chloramines in tap water will kill your filter on contact.
When you say you buy more Goldies in summer, just when do your fish die?
Im thinking the depth of the pond may be an issue; 50cm is not really enough depth to protect your fish over the harsh winter months, the deeper the pond the warmer the water stays at the bottom. Fish will stop eating when it gets cold as they go into semi hibernation which slows their system down and conserves energy until it warms up again, im thinking that they may be suffering whilst in hibernation because they don?t have the depth to retreat to which in turn could weaken their immune system.
Also, how do you clean your filter and how often? Do you have your latest water reading for NitrIte?
I wouldn?t worry about the PH too much, goldies are very forgiving of this but it does seem to be quite high. If you stand a beaker of tap water for 24 hours to let the temporary gasses escape them take a PH reading this will give you an idea of what your pond should be or there abouts. If there is a big difference, is there something that could be raising the PH of the pond? such as edging stones maybe!
The pics on my profile were taken when it was new and hadn?t yet been arranged to their liking. It?s about 18 months old now, and going great guns. Multie?s are multiplying all over the place but the punks haven?t managed to rear any babies, which is possibly down to the multies spreading like wild fire.
I have another tank which needs to be set up so I can transfer the punks to give them the room they need to feel secure, that?s a project which has been on the back burner for a while so I really should get my finger out and get it set up.
Hi simplyobsessed, I haven?t got anything to add to the great info you?ve had so far other that to endorse every word.
The majority of pet stores & fish shops are in it to make money plain and simple. The links under my signature strip have already been advocated by others on here but I would urge you once again to have a read through them.
Members here come on day after day to pick up the pieces for people who have been lead astray with the woeful lack of knowledge given them by fish shops who just want to send you away with a container, (I wont refer to them as tanks/aquariums because they are not) and a couple of cute iddy-biddy fish (babies) to put in it. We have nothing to gain, we don?t get paid to come on here, the advise is sound and absolutely free, the only difference is that we have learnt through years of experience the best and most successful way to keep fish humanely.
Ask as many questions as you like, we generally have members on at all times of day and night (those who cant sleep )