Undergravel filtration and Cory group fish

[GaryE]

I've built a few undergravel filters over the past few months, in my quest to see what works from past aquarium keeping techniques. So far, they've been great.

I understand the logic of saying that since a bottom dweller like a Cory is in contact with the substrate, and since the substrate is filter media, there could be a problem. I'm not seeing such a problem though, as I do proper maintenance to pick up mulm as you should with those filters. But the filters in question are all under a year old.

I also understand my own tendency to see what I want to see. So what direct experience can people bring to the question of UG filters and Cory catfish? Are they really problematic, or is that the usual fishlore, with no basis?

 

[Magnum Man]

it would seem the main problem would be a cross over between acceptable substrates that would work for under gravel, and the Cory's

are you using sand??? I would guess that a screen, coarse foam, or green scrubber kind of media could be used to keep the sand out of the filter plate, assuming the layer of sand would be thin ( maybe one inch or less )

Have you tried reverse flow??? I had a tank I ran reverse flow 30 years ago, and I think it would have worked great, if used in combination with a proper sized mechanical filter, like a hang on back, to remove the solids, as they don't settle into the substrate

I have cory's in my South American Tetra tank, that has a full under gravel... I use a finer, rounded edges river gravel as a substrate in that tank.. it's been running that way, with cory's in that tank for several years

 

[Magnum Man]

this is the substrate I used in my South American tetra tank...

https://www.amazon.com/AquaNatural-Substrate-aquascaping-Aquariums-terrariums/dp/B094QJXLYD/ref=mp_s_a_1_3_pp?crid=2FICW5ZHSIUYD&dib=eyJ2IjoiMSJ9.dTcnsBuOhSKyvbZpmRUHK-GwruoyK2sJ9EqO1DKfESrvo3vmP0TexkMoJ5HPsmvtqp24tFQP7B2IPRgfnOR4EMdQkaR_WZ5Z2ehTEeUXx6xWmLZllEUQS9IFWbFNpnGe5nBlqeSB3IOcFqILLFEfuglCU0cu3Kz-QiWu-LHbpPsIbQrdnqUY_uoOFcUPoTKO9vEwIQtPHL2bQgL9enXvEw.gOAfXuxE1HKfDrTx2hVUG8aleemAtuaEHL2_67K2jhs&dib_tag=se&keywords=aquarium+gravel&qid=1761655911&sprefix=aquarium+,aps,226&sr=8-3&tag=ff0d01-20

 

[GaryE]

I cheat, since I prefer two filters per tank. About two/thirds of the substrate is a fine gravel, too coarse for sifting but not by much. The other third has no piping under it and is siftable sand. What I'm curious about is whether the filtration substrate is unhealthy for Corys, or if that's simply something that is said but not experienced.

I had healthy Corys in tanks like this 30 years ago when I never worried about it, but the sample was small. and hearing from people who are on the forum and have done this might be helpful.

 

[jaylach]

Some may disagree and I don't have much experience with corys. Still, in the long past, I have had them.

Many will say that they absolutely NEED a sand substrate bit I don't agree. I think the biggest issue with gravel is that most that is packaged have sharp edges which can, and likely will, cause damage to the barbels. However I believe that smaller, polished, river pebbles are fine as there are no sharp edges. Smaller river pebbles are also not small enough to sift through the UG filter plates yet still small enough for a Cory to move around and sift.

 

[plebian]

Everything you hear about corydoras requiring a sand substrate is nonsense. Corydoras are found in environments with sand, gravel or mud, or a combination thereof, in real world environments. In most of the Amazon videos I've seen, whatever the substrate might be, it is often completely buried beneath leaf litter. While I have no direct personal evidence, I have also read at least one report testing the hypothesis that sharp edged gravel damages cory barbels that found no evidence to that effect.

Lastly, I have personally kept corydoras over a period of years using fine grade (3-4 mm) natural quartz gravel over UGFs and have seen no evidence of disease or physical damage whatsoever. I suspect the problem reported with gravel is due to the use of pea gravel, which traps food that can't be reached and therefore rots.

 

[GaryE]

The sand versus gravel issue will never be resolved. I go with the idea that the Corydoras group are sifters. They sift fine particulate matter. That's anatomical.
We can't have tank bottoms of mulm and mud mixed with sand or fine gravel, so that substrate is out. It may be natural, but our aquariums aren't. So as usual, hobbyists apply 'hacks'. Corys can feed without sifting, as can other eartheating fish like Geophagus.
Sand is found in many of their habitats, and in parts of their habitats, and it's a sensible addition IF you want to see natural behaviour (sifting). If you don't care, then the fish will live well and long over gravel. Over pebbles, I have seen problems with trapped food, but people will argue for that being fine.
MY concern here wasn't texture, but trapping. The goal of the old UG system was to turn the whole substrate into filter media, a clever goal I think we ditched too soon due partly to marketing. With the right fish, it was an effective system. I'm glad to see @plebian 's post address that side of things, especially because it said what I wanted to hear!
I'll keep following this one in case of other experiences. We usually say 'other opinions', but this is experience based, not opinion.
Corys move down, and up rivers, streams and brooks. The substrate and where the food is will vary. When I was a fish fascinated kid, I thought tropical streams would be different from the ones I explored locally. The plants, microscopic life, temperatures and fish are, but if you travel down a couple of kilometres of most undamaged streams where you live, you get a picture of the substrates.
I often read UGs harmed bottom dwellers, and because they trap debris, exposed them to high bacterial loads. I also know UGs were the advanced filter of choice in the balanced aquarium myth years, when water changing was seen as messing up the system, a la father fish. I'm thinking that with current maintenance regimes of changing at least 25% per week, and using gravel vacs (a relic of the UG period of the hobby), these old filters should be very good. But I don't want to harm my corys.

The bags of gravel I used were labelled as sand. They're not inclined to pack, but aren't siftable for a little cory. They got thrown around a lot when I had big Geophagus. I think sometimes when we debate sand vs gravel we can be talking about the same thing. But when we talk about filtration substrates vs fish health, then we're debating something different.