Big fish or little fish?
Big fish or little fish? - "I want to see them grow" is the most often given answer as to why people select small Discus ( 8cm or less ) and that along with budgetary considerations are fair enough, but,
Remember - domestically bred Discus are all hybrids / mutations and do not live in the wild - when buying small (8cm or less) the fish may not always grow into the adult description, and some may simply just not grow.
Some of the new less established strains are more prone to slow growth than the established Turquoise type fish, so other than genetics what else will affect a small Discus's growth?
The answer is numerous things, some we control some we don't, the water, is it high in nitrates, phosphates, low in mineral content ? Medications? has the fish been subjected to medications ? Diet? Stendker Discus are fed exclusively on Stendker Goodheart at the hatchery and most resellers, have you changed their diet, feed bloodworm, flake, granules ? It is my opinion that many health issue with Discus stem from under feeding with the correct food, Discus require a lot of high quality food -1.5g of Goodheart per fish per day.
"I bought a number of small fish, most are doing great, but one is not growing" is a common observation
.. Discus are cichlids and there will always be a pecking order and that means "one at the bottom" (discus are not nice to each other sometimes) the fish at the bottom of the pecking order may well feed just fine, but doesn't really stay with the others, often hides and looks dark and sickly - Is you aquarium too big ? Discus like the security of many other Discus around them - 6 / 8 small Discus in say 400 litres is not so good, it is recommended to raise small fish in a small aquarium - and move them once mature. Not all domestically bred Discus, or wild caught for that matter are perfectly round and fish that don't grow sometimes take on an elongated shape.
Some people are lucky and all their fish grow to 17cm in 18 months, others less so and you either are happy to take on the challenge of small fish, or by pass the early stage of the fishes development and look to 10cm or larger fish.
From 10cm what you see is what you get, but if patterned the fish this can change as it reaches maturity.
If you choose smalll fish , and most people do, here are a few ponters that will help not hinder their chances of normal growth
1/ 180 litre aquarium, 10 fish.
2/ Prepare the tap water with a HMA filter, do not use RO or deionised water.
3/ Mature filter - 29 /30 C - 30% water change once a week
4/ Keep nitrates and phosphates in check
5/ Diet, 1.5 g of Goodheart per fish per day, preferably spread over 3 feeds - avoid dry food.
6/ If starting out, stick to the more established strains
7/ Avoid medicating small fish unless absolutely necessary.
9/ Be proactive, not re reactive to any issue, watch you fish daily for any changes in behaviour.