^^ Prime does the same thing.
My levels are:
Ph - 6.0
Ammonia - 1.0ppm
Nitrite - 0ppm
Nitrate - 10ppm
Some of the reviews on the tank do suggedt similar problems with the filter, but I didn't think my levels where bad enough to kill off my fish.
The issue isn't with the filter, as long as its moving water over the media its fine. The problem is that you have lost a shed load of bacteria due to the tank and substrate change. You need time to build it back up.
Is that PH correct? Seems really low for tap water, almost all of the UK is hard/alkaline (7.0+).
So water changes & dosing with Seachem stability and prime it is then...
This, and do nice big water changes every day, the cycle will come back much quicker this time. It will likely only take a few days if you saved all the old media.
How do you guys feel about me adding another filter, running both while it cycles and then removing the stock I believe interpet filter?
That's ok but there are drawbacks and it comes with its own issues.
There could be too much flow which is now blowing the fish around the tank.
You'll need to run both for weeks though, most of the bacteria will be concentrated in the old filter. As soon as you remove right away as you are suggesting you'll be back at square one. Running them side by side for weeks will mean the bacteria slowly colonies the new filter but again you'll likely trigger a mini cycle as soon as you remove the old one.
Most would recommend just dropping the old filter media into the new filter but that also has its own issues in small internal filters. Most of the time the one set of media doesn't fit into another filter. You end up having to use part old and part new and is what I suspect happened this time. If you cant fit all of the old media in, you'll risk triggering a cycle.
There is no perfect solution, you always risk triggering a cycle when you change the filtration. You can only mitigate the risk and not completely remove it.