I'm volunteering at a seniors home to fix their tanks and both their 25 gallon and 15 gallon, have green water. They've already tried covering the tank with newspaper for a week but it hasn't worked out. The tanks are not anywhere near a window either.

Specs for the tanks:

25 gallon:
tetra whisper filter ( planning on getting an aquaclear)
ammonia:0 nitrite: 0 nitrate:0 (or almost nondectable by api test kit)
Idk why nitrates are so low, the tank has been setup for years as it seems
pH: 6.6
5-7 male guppies

15 gallon:
Tetra whisper filter ( planning on getting an aquaclear)
ammonia:0 nitrite:0 nitrate:0 ( same as the 25)
pH: 6.6
1 goldfish ( everything else died.)

The situation:
The seniors home management wants more fish but everytime they add a batch they slowy die off. I believe it's b/c the filter atm can only handle the current small bio load, albiet very well since nitrates are pretty much 0 or the bacteria are starving...idk, but once they add another batch of fish the filter can't handle the increase in bio-load so the fish start dying. Atm, I suggested for them to gradually add only 2-3 fish at a time in order to grow a sizable bacteria colony for more fish. They do not want to completely start over and re-cycle.
Thoughts?

To get rid of green water in my outdoor 55 gal, I used a UV sterilizer and a power head with a DIY cartridge (water bottle with a mix of fiber material and ceramic). It took about 3 days to get the water clear.

to get rid of green water, UV sterilizer or diatom filter.

the lighting period is probably too long if the tanks arent near a window.

Just do several water changes thats the least expensive way , also congrats on your volunteer work.

Buy a $25 UV sterilizer from Ebay. It will clean the tank up in 3 days flat.

In my experience water changes don't solve green water, it just comes back.

Algaefix for $8 will clear that up right away

I'm wondering if the green water is acting as a sort of algae scrubber and that's why your nitrates are 0... might be something to keep an eye on if you get rid of it with a UV or anything.

Do both of the tanks have GW?

I'd ask for everything that has been done to these tanks over the past 2 months.

I'm very suspicious of 0ppm across the board (that with your test kits or theirs?)

Something isn't right. Especially in that goldie tank, which should have pretty high nitrates unless someone just did a 100% water change (I'm wondering if someone recently cleaned everything and disrupted the tank cycles?)

How often are you out there to test?

True, but if you have live plants in the tank it will harm them as well.

This is very likely exactly what is happening.

What's GW? Sorry I'm not good with acronyms. Another issue is that the maintenance guy who does the regular water changes isn't the clearest guy on the whole nitrogen cycle aspect. He's the more old school fish+water=fully set aquarium type of guy..but I'm trying to teach him. He keeps going on about the importance of pH but I'm trying to make him understand that stable is better. Anywho, there could very well have been some things that were tampered with and he can't recall from memory. He comes to change the water every week. I assume he changes out a ton of water for there to be near 0 nitrates. The test kit that they are using is the API master kit that I bought for them...it's practically brand new. I even triple tested all the basics and it was 0 across the board...yet the fish lives...

I'm there tmrw and maybe once a week to check up on things.

Can you explain what you mean by algae scrubber? I thought green water was just a floating type of microscopic algae?

I feel for you! In your efforts to do a good thing, you may have a can full of worms!
While you and a lot of others may see that the tanks need real help, the folks who have been doing it may feel different. Oh my, the politics of it all?

Meanwhile I might suggest going very slow. Things could be worse. A few of the fish still live so it may take some time to get to the bottom of what is really causing the deaths. The cause will likely be several small things. Maybe just as simple as too many cooks in the kitchen? If you are sharing the work with several others, this may be difficult.
I might suggest trying to test the knowledge of some who work the tank and try to sort out if that is the cause of the deaths. Maybe not using dechlor on water changes or not watching the temperature?

Thanks but I enjoy a little challenge. I'm actually there to sort out their fish tanks, their turtle tank, and their pet birds (all of which I have at home). So yes its a can of worms but it helps me to learn some new things too . The guy who was in charge of fish tank maintenance before I showed up was the only guy in charge of that stuff. He maintained it OK I guess but things started going south and they asked me to help. So now its just him and me.

'Scrubber' is a general term for any plant (including algae) that removes nitrogen. In this case the algae scrubber is removing the nitrogen (ammonia, nitrite or nitrate), keeping it low between water changes.

I would swap fish. Put the Gold in the big tank, no other fish.
Put the Guppies in the small tank, and even then you could double the population, as long as you stick with males only.

If they want to raise the population faster then do this:

1) UV sterilizer to kill algae. Do a pretty big water change, or a couple of them to remove the dead (decomposing) algae while the UV is doing its job.
2) When the algae seems gone continue to monitor the ammonia, nitrite, nitrate for a few days. Do water changes as needed.
3) A few days after the UV has been removed, the algae is dead, and the ammonia, nitrite and nitrate have settled down, add some bacteria from a bottle that contains Nitrospira species of bacteria. Do not waste money on anything else. Read the instructions, and do not do any water changes for a few days. Let the new bacteria get a good hold on the filter media. Make sure the person who does the regular water changes understands not to do them for a few days. (Or, if you know what day he comes in, do this the next day, knowing he won't be back for a week)
4) Add fish within 24 hours.

UPDATED PLAN (with management):
So I went there today and since a lot of their equipment is old and outdated they've decided to get some new stuff.
1. Aquaclear HOBs
The plan will be to cycle the bio media from the new aquaclear in the old filter (with the old filter insert) and put the bio media into the aquaclear after a month or so...gradually converting to a newer filter. The tetra whisper that they are using is a carbon filter only...
2. Fish will slowly be added and the maintenance guy will test ammonia, nitrites, and nitrates every 2-3 days and record it. He will update me by emailing me results....that is if he remembers how to do the testing.
3. Management wants to try Hagen's Green-X phosphate remover and cover the tank again to get rid of green water.

Thoughts?