Help fixing parent's fish tank, and fish...low pH and high TDS?
I'm helping my older parents look after their fish. (Questions at the bottom).
They have a medium sized tank (AquaStart 500, 65L capacity), with three small fish in it (apologies for the photos, I'm using a low-end camera-phone, was hard to get a good photo):
(My father said there was meant to be four fish, but one of them lost their tail - but I've only seen three since I've arrived - I think the fourth might have not made it 🪦)
I noticed there was a lot of this sludge at the bottom of the tank - I'm assuming that's decomposing food/fish waste?
Orange sludge at the bottom of the tank
I thought I'd do a water change - I pulled out about 2 buckets of this out - there's still more gunk in the tank - but I was worried it'd stress the fish out if I got out more water.
Orange sludge water I sucked out
I did test the water using a cheap water tester I had (normally used for plants) - the pH was low (4.5 - 4.9), and the TDS in PPM was really high (> 900 ppm):
Is there anything you can tell from those readings? (There aren't any aquatic plants in the tank).
There's also a bunch of these snails in the water (> 20 or so):
I thought they were some kind of parasite that'd hitched a ride in - but my father said they help clean up the tank?
I do have an RO (reverse osmosis) unit, that I use for plants - I was thinking of using that for the water change.
This is the existing filter in the tank:
I'm not sure it's doing a very good job - but not sure if that's just a maintenance thing, and it needs to be fixed/cleaned/serviced? There's this weird thing, where if you turn it off - and try to start it again - it won't suck up water via the tube on the right. You need to manually pour water in the gray canister on the right, and sometimes manually turn the pump impeller....
Questions:
- Can anybody tell what sort of fish they are? (Yes, the photo is very low res).
- What does the low pH (4.5-4.9) and high TDS (900+ ppm) reading mean?
- Is it safe to use RO water as-is to replace during the water change? (roughly 25% of the tank's water). Do I need to add back things, or safe to use as-is? (I tested, the RO product water is close to 0 ppm).
- What exactly are these snails - and are they safe to leave in there? Are they good for the fish/fish tank?
- Any advice on what I can do to fix/service the filter unit?
- Let me know if there's any other advice or suggestions you have
Posted Dec 29, 2025, 3:00 AM UTC