The Rio 125 is 81 cm/32 inches long x 36 cm/14 inches front to back x 50 cm/19.7 inches tall - I used to have one. Though that 50 cm tall is right to the brim, because the lights sit below the brim the water surface has to be several cm below the brim.
Denisons barbs need a much longer tank I'm afraid.
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As does the red tailed black shark
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Cherry barbs are fine in this tank. Make sure you have more females than males - females may be less colourful but males spend their time chasing females and each other. I have a shoal of these and one of the males chases everything in the tank, and continually encourages females into the plants. One egg made it to hatch, and I now have another half grown male in the tank.
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The tank is just about big enough for pentazona barbs (5 band barbs) and they are compatible with cherry barbs. If you get them, don't confuse them with tiger barbs (which are notorious fin nippers)
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Both cherry and pentazona barbs need a temperature of 20 - 26 deg C so around 23 to 24 would be best. They are both shoaling fish so at least 6 of each, preferably more.
Gold barb - if you mean this fish
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They do really need a bigger tank. And they need slightly cooler water than the other two barbs.
Most of the other commonly available barbs need bigger tanks I'm afraid.
Colourful catfish - there are some colourful varieties of bristle nose around these days, for example yellow. There's a lovely photo of one in this contest thread
POLL HAS ENDED Loricariidae :fish: :fish: :big_boss: PLEASE VOTE! We have 9 awesome Loricariidae entered into August 2023 Fish of the Month Contest. View all the entries and descriptions below and then go to the top of page - click on your choice for FOTM and then click the "cast" button...
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beware though, many of the pretty catfish need big tanks.
Colin mentioned hardness. Scottish Water's website can be hard to navigate. First you have to find your supply zone, then download a pdf for that zone.
First enter your postcode here
Water Quality Search
www.scottishwater.co.uk
and make a note of the "site name".
Then download the 2022 Water Hardness Data pdf from here
Water Quality
www.scottishwater.co.uk
and look for the 'site name' you found on the other page. You need the numbers in the "hardness in mg/l caCO3" and "German degrees" columns. Post them here.
Just to warn you - most fish store workers haven't a clue, they make things up to get a sale. Never ask them for advice or believe anything they tell you until you have researched it yourself. For example, I bet they didn't tell you about cycling the tank before getting fish.
Fishless cycling https://www.fishforums.net/threads/cycling-your-new-fresh-water-tank-read-this-first.421488/ Silent or plant cycling https://www.fishforums.net/threads/planted-silent-cycling.470400/ Fish-in cycling...
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