Green-ish water?

My 30 gallon tank is home to one fan tailed goldfish & one koi. Each week, the water turns a slight green-ish color. I have 2 HOBs running of which one is an Aquaclear 500 and the other is a slightly modded Marineland Bio wheel filter. Slightly modded as instead of a cartridge, it uses floss & I have 2 pot scrubbers tucked in the bottom.
The Aquaclear has 2 bags of bio max along with the sponge & floss.
I do water changes each week.

Now, my 130 gallon, it never sees that color water as it is always 100% clear. It uses an XP 3 & a home-made 10 gallon tub filter. (10 gallon as in the size of the tub itself). It is loaded with lots of pot scrubbers, floss & sponge whereas the XP is pretty much a typical set up for it. That tank is home to one big, stunted ID shark & 2 baby IDs, 2 big common plecos & 4 oto cats.

My question is this, is the green water due to the fish themselves or a lacking filtration on the smaller tank? This tank does get some brown algae that I scrub off when I notice it. Would a Xp 1 be a better filter to use instead? I did see one on Kijiji for like $40/$50.

I think it is the goldfish...they seem to be algae producers when I have mine in for the Winter, even with poor lighting.
I LOVE my UV sterilizer for keeping it under control!!  A new SunSun brand 15 watt is $35 -40 on Amazon...same as the filter!!!

Is the 30 gal getting any direct sunlight?

Steve

Goldfish produce a lot of waste which can quickly ensure an imbalance in the nutrients and thus algae.

I add potassium to the tank which has goldfish and I put in some potho and other plants.

Now even with sunlight coming from the window I only get brown algae.

Steve, none at all.

So then it's nothing to do with filtration or lack thereof?

Goldfish/Koi are dirty fish that produce a lot of waste that can add to Algae growth.

What kind of light do they get?

Just as a comparison, I have a 40 Gal in my Wife's office that has an Oranda, a fantail and 14 comets, some are now almost 4". it is way over crowded, but I am tight on space right now now and have no where else to put them. That being said I do a 50% water change every Sat and it is filtered with a single Aquaclear 50. All is good and no algae.

By the way, what's the long term plan for the Koi?

Steve

They do not get direct sunlight as they are across from the balcony towards the front door on the 23rd floor.
I do half water changes each weekend as well.

As for the koi, don't know yet.

Goldfish are very great at creating algae - they're even used to culture green water in research labs!

Does the tank have a substrate? The water gets much more green in bare bottomed tanks.

It has an inch or 2 of multi coloured pebbles on the bottom.