Larger tank, moving up in size – Discus, Oscars and Tigers too!

Larger Tank Considerations - 120 Gallon Aquarium

We have had many comments about various articles and one particular enquiry raised so many points about moving up to a ‘larger tank’ that we thought it would be a good idea to try to combine our answer to include the advice in those articles.

The Question

120 Gallon TankThe main question centered on getting a larger tank.  We were asked about a 120 gallon tank and whether to stock it with Discus, Tigers or Oscars etc.  The enquiry went on to ask about using external filters, probably EHEIM Pro or similar.  They also pointed out that the water they have is on the hard side with the pH about 8 and they did not want to be bothered using RO water and CO2.  They wanted a low maintenance tank and were happy to have fewer fish to make it easier to look after.


So, what would we suggest if they were to stock the tank with a range of fish such as Discus, Corydoras, Tetras and Tiger fish or maybe larger species like Oscars? 

Corydoras sterbai
Corydoras sterbai


FTF Response:

We have linked to the relevant page for more information.  Where you see an underlined word or phrase.  For example, 120 Gallons highlighted color links you to the following page ‘120 Gallon Aquarium – is it right for you?’  if you click on the link.

The 120 Gallon Tank – the ‘larger tank’

EHEIM Classic 600

120 Gallons is a lovely size tank for anyone wanting to move up from smaller beginner tanks.  The top of the EHEIM Ecco Pro range, the ‘300’ quotes aquarium volumes between 160-300 litres which is only 42-80 gallons, so this would not work.

The Filter

If you are stuck on using an EHEIM filter, I would consider the EHEIM classic 1500XL, this is suitable for aquariums from 80 to 396 gallons – so easily covers the needs of a 120 gallon tank.  Even the EHEIM Classic 600 goes up to around 160 gallons.  These are good value for money, they are very quiet running and have low energy consumption.  Usually they are robust and should perform well for years.  Generally they are good quality filter, so in response to the query, we are happy to run with this filter.

We have reviewed many filters on in the fish tank filter review page – and the EHEIM brand is there too.


The Planting & Substrate

Vallisneria Americana Gigantea

If you are going for the smaller fish selection, such as Discus, Corydoras sterbai and maybe some bleeding heart Tetras, you could plant the tank with Vallisneria Americanagigantea’, which are easy to grow and get large, so will suit the tank’s dimensions, and although they are heavy feeders, they don’t need CO2.  You might even include a few Tiger plecos for algae management.  

Decorate the tank with quartz sand over a nutritious substrate (there are many good substrates to choose, click here for the best sellers), perhaps a piece of bogwood and some Catappa leaves, the fish would be very happy in this lot, and the plants would help control the nitrate – the main reason for changing water.


The Water Chemistry

The bogwood and the leaves will acidify the water slightly.  To maintain the right water chemistry be careful when you do a water change. Any new tap water as stated in the query being approx. pH of 8 will cause the pH will shoot back up.  Some first layer substrate fertilizers also contain peat, again aiding the lowering of pH in the water.  For more information on Water Chemistry see our latest article ‘water hardness matters – chemistry made easy’.

Tiger Pleco
Tiger Pleco

The Tiger Fish tank mates

The Tiger plecos will require high oxygen levels and of course there is a risk of them eating the plants, but the tank is reasonably large and if you keep them well fed with fresh greens and prepared dry algae-based diets, they should be fine.  Whiptail catfish might be a better alternative.


The Oscar Option

The Oscar (cichlid) option would be a great choice for this size of tank – not too many though as they do grow to about 12 inches and quickly.  Note that they will grow one inch a month until fully grown – they won’t like a cramped tank.  The ideal water conditions are in the range of 74-81°F, 6-8 pH, and 5-20 KH.  Oscars have a variety of common names,  including the tiger oscar, velvet cichlid, and marble cichlid.

Astronotus ocellatus

For Oscars décor, go for large pieces of bogwood and sand.  The problem here is with the plants – the cichlids might eat or destroy them.  Live plants are unlikely to be eaten, but they’re still not safe. Just like the decorations, plants may be uprooted while they dig through the substrate. This species will dig around objects when looking for food which can dislodge them.  They will uproot plants and decorations during this search, everything in your aquarium should be fixed down. Anubias spp, attached to the bogwood décor may be okay, but don’t be afraid to omit plants altogether and instead control nitrate with nitrate absorbing resin media in the filters.

Oscars love caves give them somewhere to hide within their own territory!

I would change the tank mates though, matching them up with other large, ‘peaceful’ South American cichlids like Severums, Uara and Hoplarchus.  Oscars are territorial, so adding them to your tank can be risky.  They aren’t afraid to attack other fish and will do so if a fish encroaches on their territory. 


The Decision on Fish Selection

It’s not easy to call this one – it’s a tough choice.  The Discus tank would produce less waste, so less mechanical media cleaning and changing, but Discus are far from mess-free, there will be some cleaning required.  The Oscars are possibly more worthy of the larger tank because they grow over 12 inches.  Considering also that Discus get spooked in large tanks too, although planting should help to settle them. 

The final choice must be by the tank owner, as the aquarist is the one to get the most enjoyment the selection has to suit you!

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