I suspect you lr wasn't cured
cyno
Interesting piece I found on the internet
Because this type of algae does not attach well, it can be peeled off and removed by light siphoning, with larger floating pieces being removed with a net, or turkey baster.
Carbon Dioxide (CO 2 ):
Low water flow or movement throughout the aquarium produces carbon dioxide (CO 2 ), which algae consume.
slime algae "consume" nitrates, often when aquarists perform nitrate tests, the readings come up as normal. Don't be deceived. If you were to remove the algae temporarily, in all likelihood you will see a rise in the nitrate levels in the aquarium. It's like a catch 22. The nitrates have actually been there all along, but unreadable as the algae is feeding on it, therefore the nitrates "appear" to be in check. This applies to many other forms of algae as well!
Looks like you have very high nitrates which are being eaten by the cyno, thats why you are getting the reading which appears to be 10/20 when in reality it is much higer
Don't add anymore cuc - they will only die
don't add anymore food, try to peel off what cyno you can, you don't have any bio balls or foam in do you - if you do, get them out. turn off your lights and increase the flow -what flow have you currently got? Are you using a protein skimmer ( just looked, you are not)? Have you got one, if so turn it on
Do much larger water changes
Seffie x