suggestions for getting rid of them?
Look like aphids. Possible treatments (I don't have a magic bullet)
1) Remove plants and wash under running water.
2) Remove plants and spray with water/washing liquid solution as per house plants and then wash under running water.
3) As in (2) but with Neem oil added to the mix and again wash under lots of running water.
4) Place plants in a deep container and fill container with carbon dioxide and cover with clingfilm to keep the gas in. (C02) is very toxic to insects.
5) Rinse plants in the tank so that the fish have insects to eat. If they like the food then you have a free live food culture.
Personally I would go for (4). You can make CO2 from washing soda, from a hardware shop and citric acid powder from a supermarket. Put a little water in a bucket, put plants in the bucket. Put a jam jar or similar with the dry ingredients in it in the bucket. Add water to the jam jar, instant fizzing, and cover with clingfilm.
Repeat gassing frequently. Plants should love it.
At the moment I am trying (3) on emergent anubias leaves which have a light mealy bug infestation. I use a small plastic scent sprayer and mask the tank surface with one hand while spraying with the other. The plant is large and would be difficult to remove without a lot of disturbance.
Never ending with conservatory plants. Some would favour a biological control to aphids and similar but I am wedded to chemicals. Each to their own.
Good luck & I hope things work.