Cichlids Not Eating

Okay, I have a pair of ZZ flowerhorns and a juvie male Umbee in a 150g tank with a 2260LPH canister filter packed with jap mat and coarse sponge with UV sterilisation and 3 massive sponge filters with a divider between the flowerhorns and umbee. Previously the flower horns have been eating brilliantly but they went off food for no apparent reason so I decided to add a baby umbee (around 7") to the tank (with a divider, of course) but now all 3 fish aren't eating and are rather skittish. Water is perfect (0 ammonia, 0 nitrite, about 25ppm nitrate, pH around 7.8, temp. at about 26-27*C), I've tried literally every dry or frozen food I can think of (Hikari Cichlid Gold, Cichlid Staple and Sinking Carnivore Pellets, Hai Feng Fast Colour, NLS Thera+A Large Fish Formula, frozen shrimp, fish fillets) and even live crayfish. What now? I also have a wicked big bag of Shori High Growth Formula Koi Diet which has basically the same nutritional profile as Massivore and I am not going to waste $150 of food if they aren't eating.

And yes, I know the Umbee is going to get 20"+ and I'm planning a MUCH larger tank but in the meantime, he's a dither for the flowerhorns.

Stomachs aren't sunken in and while they were still eating, they did big healthy brownish coloured poop but now they aren't eating, there's not really not a whole lot of pooping happening.

Well, the flowerhorns aren't aggressive, more curious and the umbee goes between attacking the divider and hiding in the ceramic log.

I might add, I got the umbee 2 weeks ago and he hasn't eaten since I got him and the flowerhorns have had maybe 10 medium sized Hikari Cichlid Staple pellets each in a total of about a month. All 3 have previously been complete pigs (or so I was told about the umbee) so this is a little unusual...

The umbee was from a very reputable source (thousands of dollars of rare fish and rays in the same system) so I'm doubting the parasites as he also bred and raised the umbee but treating it is worth a shot.

I believe a 150 is plenty of room for a pair of flowerhorns with a dither.

Yeah, flower horns are funny like that. These 2 get along remarkably well and barely ever fight and when they do, it's usually the female in charge.