You likely have a high concentration of organic waste which the algae is taking advantage of. The overflow box is showing evidence of algae so i'd be curious to ask how long the lights are kept on. Planted tanks can safely run on 6-7 hours of light a day.
Considering the plants are in pots, you can take them out and dip them in peroxide for 10-15 seconds then return to the tank.
Given that you have a 30 gallon sump, the water volume in the tank is more than sufficient to sustain your two goldfish and loaches. It's true that a smaller species fish may be happier in a 40g breeder but your tank is fine as it is.
Maybe its worth taking a look to see if the sump or overflow needs cleaning.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
"I am Groot", the faithful protector