Substrate Questions for Corydoras

[darrylzuk]

I have a moderately planted (will skew towards heavily as the plants grow in) 29g tank with Flourite black as the substrate. It's fully cycled, and I'm in the process of planning what fish to add as a community tank. I'm interested in getting a school of cory cats to occupy the ground floor, but the internet seems to be split on whether Flourite is okay for them. From the reading I've done, there's a group of people that say most health problems and barbel issues result from bad water parameters, or a dirty substrate. This group suggests that Flourite is fine, and there are lots of anecdotal accounts of people keeping healthy, breeding schools of cories on Flourite for years and years without any problem. Additionally, Aquarium Co-Op claims "our CEO Cory McElroy visited their natural habitat in the Amazon and found the substrate to be quite sharp", suggesting Flourite might be okay(?). The other group says Flourite is too sharp and abrasive and you absolutely need sand for them to be happy and dig around like they would in nature.
At the moment, I have a window of time where I could install a beach of sand for them, but I'm trying to weight the benefit of doing so. If it would be a significant quality-of-life improvement for them, I'll absolutely do it. But if they're more likely to be "meh" about it, I'd prefer to not tear up a portion of my tank to install it.
Secondly, the internet seems divided on what sand is best as well. I would prefer to keep the black/dark aesthetic of the tank, which lends itself to one of the following:

• BDBS;
• Tahitian moon sand, which seems to be discontinued;
• Imagitarium black sand;
• Flourite Black Sand; or
• Fine Grain Volcanic Sand, like this from Etsy

BDBS is generally well-reviewed in most places, and is inexpensive, though I've come across a few posts who say PFS is the ONLY option, which doesn't come in black. So, assuming I'm going to make a beach for the cories, which sand is best? Again, if PFS is really the best and will make them happier than a darker sand, so be it.
Thanks in advance!

 

[CassCats]

Play sand 100%
But if you are set on darker sand, don't go pure black, go almost dark sooty grey as pure black can also be stressful to them as its unnatural.

But no, fluorite isn't good.

True, their substrate may be sharper and varied in the wild, but its not a closed system in a river. Impurities are able to be washed away down the current or fish are able to avoid it.
But in a closed environment like a fish aquarium, impurities and toxins can accumulate, especially in gravels and substrates where there are gaps, which is why gravel is not recommended for corydoras. It is easily prone to harboring pour bacterial which can prove hazardous to corydoras barbels. Sand, on the other hand, isn't as open for bad bacteria to grow as rampantly, giving less room for infection.

 

[Byron]

This question will have varying opinions from members, and that is fine. However, to answer the question responsibly one has to know the actual facts, and as you have seen from your research so far there are few who do.

First, that comment about Cory McElroy finding a habitat with sharp substrate is misleading because it is deliberately perverting the facts to come up with a preferred answer. All species of Corydoras in South America live over a substrate of fine river sand. Period. There are obviously going to be habitats with mixed substrates, due to geological factors, road building [one idiot actually claimed this dumping of rough rock in a river was proof that cories had evolved over this substrate], or whatever...but in all of them there are areas of river sand and this is where the cories filter feed. And if you research how they filter feed, you will immediately see the critical importance of the sand used.

I have discussed this issue with individuals like the explorer Heiko Bleher, and Ian Fuller who owns CorydorasWorld, and has been collecting cories in South America for several decades, and raising them. Anyone who thinks they know more about the subject than experts like these two should not be in the hobby. We are maintaining living creatures and it is our responsibility to do sufficient research and then provide accordingly. Listen to those who do know, and have nothing to gain financially or otherwise from their advice.

None of the so-called plant substrates are acceptable for cories. I had Flourite Black some years ago, and within just a few days it ripped off the barbels of the dozen cories, and one panda even had a corner of its lower jaw sliced off. I moved them to another tank with play sand as it happened, and all recovered and now some 7-8 years later are still with me, even the one with a lop-sided lower jaw. The plant aspect of Flourite is a myth as well; I left it for two years (having removed the cories) but plant growth was not noticeably better. Waste of money.

The best sand is aquarium river sand. This sand is tumbled over hundreds of years to be non-rough, and it is what cories live over in SA. I have found Quikrete Play Sand to be the same, and in eight years with this in all my tanks I have not seen any fish or plant problems. It is natural, safe, and the most highly refined commercial sand.

No industrial sand is "safe," no matter who uses it and says differently. Look at it under a magnifying glass. Anyone who cares about their fish will provide the best.

 

[AdoraBelle Dearheart]

I would trust @Byron on this.

Look at this way; You know for sure that a very fine sand like the river sand Byron mentioned is safe. Bear in mind that cories will spend most of their day filtering this sand through their gills....

But flourite or gravel or anything else - you know you're taking a gamble, and that there is a risk it could injure the fish.

To me, that makes it a simple choice.

 

[darrylzuk]

So it sounds like play sand is the best and most responsible option. My follow up question is would I need to re-do the whole tank, or would the idea of creating a sandy area/beach be okay, and if so, what percentage of the bottom of the tank need to be sand?

 

[Byron]

So it sounds like play sand is the best and most responsible option. My follow up question is would I need to re-do the whole tank, or would the idea of creating a sandy area/beach be okay, and if so, what percentage of the bottom of the tank need to be sand?

There are two aspects to this. Providing a decent-sized area of soft sand would allow for the cories to filter feed as they expect to (this is programmed into their DNA as part of their inherent needs/behaviours) but what about the other rough area? Food will get between the grains there too, and the cories will understandably try to get it out. If this is on the rough side, trouble. Cory barbels are designed to sense out food in soft substrates of sand and mud. This is what they all live over, either or both.

Filter feeding is more than just gill issues; when cories take in a mouthful of sand, they filter feed the bits of insect larvae or whatever and swallow them (cories have no teeth), Some sand is swallowed too. Imagine the damage rough sand could do not only to the gills, but the inner mouth, stomach, intestines. Look at the grains of gravel like FLourite under a magnifying glass, then imagine swallowing it.

Different substrates do not look natural in an aquarium, and they draw the physical space closer making it seem even smaller. By this I am not referring to a basic sand substrate on which there are some rounded river rock pebbles; this is certainly natural. But a dividing line (necessary or the substrates will mix) to have distinctly different substrates always looks artificial to me. I admit this is a personal preference, and not something directly detrimental to the fish so far as appearance is concerned, but the detriment in the non-sand material is serious to the fish.